The members of Valkyrie Unit Echo 3-6 listen in horror at the emergency distress calls coming from the rest of their fleet. Their colleagues tell of monsters ripping them apart and warn others to keep clear of the asteroids as they're protected by magnetic bombs. The fleet has evidently been lured into a trap and half their ships are on fire. When the monsters move in for a second attack, Lieutenant Beth Stokes orders the transmission to be switched off. The crew can't believe their enemies are prepared to attack hospital ships as well, but Stokes tells them it's standard operating procedure to move onto weaker targets once you've eliminated the strongest threats. Sergeant Tahira Khan wants to move in closer in case some of the ships jettisoned life pods, but Stokes refuses as she knows their enemy doesn't leave survivors. In any case, they've already sustained too much damage themselves. As they argue, the bridge of the ship is rocked by a huge explosion.

The Black Dalek arrives on the bridge of his command vessel and is informed that 73% of the enemy fleet has been destroyed. They detect a mass exodus of escape pods and the Black Dalek orders their patrols to exterminate them. They receive an incoming communication from Lt Stokes telling them that their fleet is made up of civilian transports carrying refugees. She pleads with them to show some mercy and break off their attack, but the Daleks consider all humans to be their enemy.

Stokes order Khan to head for the nearest human outpost, which turns out to be a scientific research facility called Roarke 279. Khan insists they should stay to fight the enemy, but Stokes refuses to back down. The Daleks detect their ship escaping, but the Black Dalek decides to let them run. He knows the humans will seek refuge with their own kind and this will lead them to their next target.

The TARDIS materialises on the planet Bliss, a rare sanctuary for flora and fauna in a galaxy riven by war. On their way here, the Doctor had likened it to the Galapagos Islands, but Hex and Ace are far from impressed by the grey creepy trees covered in huge thorns and the smell of burnt silk and static electricity in the air. The Doctor is sure they've landed in the right place, but something is obviously wrong. Ace jokingly suggests they go back to the TARDIS and is surprised when the Doctor takes her seriously and says it would be safer for them inside. He clearly doesn't want them involved and walks off into the distance, promising to come back soon, but Ace and Hex realise things must be really bad here, so they insist on going with him.

The Valkyrie ship breaks through the atmosphere and heads for the far side of the planet where Roarke 279 is located. They've used up all their reserves so although they have enough fuel to get them there, they won't be able to leave again.

The Doctor identifies the creepy grey trees as Ironweed, a pernicious form of plant that feeds on other vegetation and has mercury for sap. It normally only grows to the length of a man's arm, but these trees are 20-30 feet high. Hex has noticed they're growing in rows, as if in a plantation, but the Doctor says that kind of deliberate industrialisation shouldn't happen on Bliss. Hex gently touches one of the thorns, but it slices through his hand like a sword. The Doctor uses his handkerchief to bind the wound, but then Ace calls out and shows them a clearing littered with at least a dozen dead bodies. The victims have been shredded, but a piece of material identifies one of them as a member of the medical staff from Roarke 279. The Doctor believes these people had been herded here before being attacked. Suddenly they hear the roar of an engine and a spaceship with military markings flies overhead. Ace identifies it as a short-range escort fighter, which makes its presence here odd.

The ship comes in to land beside the research facility and the crew disembark. Stokes orders Khan to get their wounded troopers ready while she makes contact with the staff here. She uses the comms device on the main entrance and requests permission to enter. The voice of Professor Toshio Shimura, a middle-aged Japanese man, responds but tells their next military inspection isn't due for another month. Stokes tells him it's a medical emergency and her troops need treatment, but Shimura tells her admission is impossible as parts of the facility are under quarantine. Stokes reminds him that Roarke 279 is funded by the military and warns that he'll suffer serious consequences if he refuses. Reluctantly, Shimura agrees and the huge main door slowly opens.

The Doctor orders Ace and Hex back to the TARDIS, but they both refuse. He knows he'll need some equipment from the ship to analyse the wound patterns on the victims, so he agrees to debate the issue with them on the way back. Then Hex hears a humming in the distance and they see what appears to be a cloud coming towards them. On closer inspection, it turns out to be a swarm of huge hornet-like creatures. The Doctor identifies them as Piranha Locusts and says they were undoubtedly the cause of the massacre here. These creatures are four to five times larger than normal and the Doctor wonders what could have made them so big. There isn't time to discuss the matter, so the three of them run away as fast as they can.

Shimura greets Stokes and asks her to forgive him for his earlier abruptness. Stokes knows this facility was designed to support 40 people and wonders where all the others are, but he tells her most were evacuated when war spilled in the sector and those who stayed behind to continue their research were struck by an infection that he suspects is alien in origin. A few have survived and are in isolation, but Shimura seems to have a natural immunity. He says he's close to completing his work here, but he refuses to discuss it with her as she doesn't have the proper security clearance. Stokes says the Daleks will have control of this sector within days, so once her crew are patched up and the ship has re-fuelled, he'll have to be evacuated to the nearest military base, but Shimura says he'd rather die here than leave his work unfinished.

Outside, the swarm is gaining on the Doctor, Ace and Hex and they know they can't outrun them forever. The Doctor leads them to a building up ahead, but the entrance has been sealed tight and they're unable to get inside. Ace starts banging on the door furiously while Hex presses the button and pleads for whoever's inside to let them in. The swarm is only seconds away.

In the infirmary, Stokes checks on the medical droid, Sistermatic, who is treating her troops. She wants them to be ready to leave within the hour, but Sistermatic says that although seven of the officers are 'walking wounded', the remaining three are badly injured and won't survive another move. Stokes becomes angry and Sistermatic recommends a course of psychological counselling. She asks to be shown the isolation unit and is surprised to discover the infected crew members have been locked inside to stop the contagion spreading.

Sergeant Khan contacts Stokes and tells her to come to the command centre quickly. Once there, Khan shows her a security feed of the three 'visitors' waving for their attention at the north-east entrance. Khan is certain they're not refugees from the fleet, but Stokes remembers hearing rumours that the Daleks were using replicants to infiltrate their forces and sabotage the war effort. She asks the Doctor, Ace and Hex to identify themselves, but they tell her there isn't time as they're about to be attacked by Piranha Locusts. Stokes considers switching on the perimeter security grid, but Khan says they can't do that while people are outside as they'll be cut to pieces. The Doctor demands to be admitted under the Protocols of Sanctuary, but when Stokes questions him further he admits that he just made that up. He tells her she'll face a severe reprimand when her commanding officer hears about this unless she opens the door immediately. Eventually the door is released and the Doctor and his friends bundle in just as the insects swarm into the area.

Ace and Hex are angry that Lt Stokes took so long to let them in, but the Doctor knows she was afraid. Hex wonders how he knew this building was here, but the Doctor assures him it was just a lucky guess. They soon find themselves facing the guns of Stokes and Khan. The Doctor introduces himself and says they've come here to help, pointing out that Ace has some experience with weapons and Hex is a trained nurse. He recognises their uniforms as those of a Valkyrie Unit, an all female fighting force created to combat the most dangerous and malevolent species ever to attack mankind. Ace knows immediately who he's referring to - the Daleks! Stokes tells them the facility is being abandoned and orders them to leave, but Hex reminds her there are still Piranha Locusts outside. Instead, they offer to help with the evacuation and when Khan scans them to confirm that they're not Dalek replicants, Stokes agrees on condition that the Doctor help her make Professor Shimura see sense.

The Dalek ship enters orbit above the planet Bliss and begins a surface scan, but their long-range sensors are ineffective because they're being blocked by an atmospheric anomaly. The Black Dalek orders a sub-orbital patrol to investigate and search for humans.

The Doctor and Ace fill Hex in on the Dalek story and he's surprised that they never mentioned them before. When the Doctor likens their destructive methods to those of the Piranha Locusts, Stokes is shocked to learn about the 12 corpses in the forest outside wearing clothing linking them to this facility. Stokes tells them Shimura had claimed his staff had either been evacuated or had been quarantined because of an infection, so it appears he has a lot of questions to answer. The Doctor knows of Shimura's reputation and agrees to help her get to the truth. His only regret is that he didn't make Ace and Hex return to the TARDIS when they first arrived. He can't be certain what's going on here, but it's clear to Ace that he has some nasty suspicions.

In his laboratory, Professor Shimura prepares to give what will be the final infusion. He apologises to the subject of his experiment, but says it can't be helped as all births are painful. He performs the task, then begins to sing a lullaby to help the subject get back to sleep. Stokes arrives outside and demands to be let in, but he refuses and says he can't leave his work at a critical stage. She insists that he give her some answers, but he tells her to leave him in peace. Stokes orders Khan to open the door, but Shimura has encrypted the lock.

Suddenly the corridor is filled with a painful high-pitched sound and the Doctor warns that this is the beginning of the end. The noise stops and a Dalek voice comes over the loud-speaker system, calling for their attention. It orders all humans on the planet to surrender or face extermination. In the laboratory, Shimura's subject becomes agitated and the Professor tries to calm it down. The Dalek tells them they have 15 rels in which to lay down their weapons and deactivate all automated defence systems. Khan says that doesn't give them enough time to evacuate, but Ace knows it's pointless anyway as the Daleks will be waiting in space, ready to destroy any ship that takes off from the planet. Stokes is surprised that the Daleks were able to follow them here and is stunned when Khan blames her for what's happening. Although she gives orders for the meteor shield to be put on line, it needs Ace to recommend they fire up the perimeter security grid too, and Khan is angry that her commanding officer is having to take advice from civilians. Stokes has heard enough of her Sergeant's insubordination and gives her orders to hand out weapons to everyone and supply the Doctor's group with voice-activated communicators.

The Doctor refuses to elaborate on what he means and says very little will ever be made public about what happens on Bliss. He knows he has a role to play here but with the Daleks on their way and the Piranha Locusts outside, it's too late for Ace and Hex to go back to the TARDIS now. They hear an explosion and the corridor starts to shake.

In the command centre, Stokes and Khan monitor where the explosions are coming from and realise the Daleks are softening up the planet with an orbital bombardment before their main attack force arrives. The facility's meteor shield is only capable of withstanding one or two direct hits so Stokes orders Khan to divert all secondary power to boost it. Stokes still can't understand how the Daleks found them so quickly as she was certain she'd shaken them off. Khan reports that the bombardment has stopped, which means the Daleks will be coming for them soon. Although Stokes has only ever fought the Daleks in battle simulations, she knows their methods well because they attacked the colony where she grew up. She says that if they're lucky the Daleks will have orders to exterminate them, but if they're unlucky they'll be taken prisoner and used as slave labour. Hex is confident that Stokes and her team will be able to fight them off, but Ace knows a standard Valkyrie Unit consists of only 20 troopers and from the state of their damaged ship they're bound to have sustained casualties. The Doctor suggests they head for the infirmary, but Ace wants to go to the command centre to see if she can help there. Hex is surprised the Doctor wants to visit the wounded rather than investigate Professor Shimura's lab, but the Doctor says it's important to find out what they're dealing with first.

The Daleks attack force arrives on the surface of Bliss and reports to the Black Dalek in orbit. They instantly come under attack by the Piranha Locusts but they're no threat to the Daleks. They open fire and completely wipe out the attacking insects. The research facility is then detected and the Daleks are ordered to converge on the location and eliminate any resistance.

Khan detects the Daleks approaching and, knowing they won't risk bombarding the planet while their own troops are on the ground, she switches power from the meteor shield to the perimeter security grid. Ace arrives and offers her experience as a Dalek fighter, but Stokes dismisses her as an amateur. However, Ace spots that the lasers of the defence grid have been fixed on their narrowest possible setting to keep out the insects and suggests they widen the gaps to make the beams much stronger. Khan is impressed, but once again Stokes is lost for words.

The Doctor and Hex arrives at the infirmary and introduce themselves to Sistermatic. They claim to be on a fact-finding mission to find out how well artificial medics cope in battlefield conditions, but the drone corrects them and says this is not a war zone. They examine some of the patients and Hex is shocked when he discovers how severe the injuries are. He also learns that some of it was the result of friendly fire after one of the other troopers panicked during an attack. The Doctor asks about the staff from the research centre who are being kept in quarantine, but Sistermatic says only three of them are still alive and their symptoms are beyond its programming. The first case appeared three weeks ago and it spread quickly. They examine the patients through a window and note that the infection is eating them alive. The speed and virulence of the contagion is beyond anything recorded before and the patients have lost half their body weight in just two days. Hex is angry that they've just been locked away and left to die, but Sistermatic says it was necessary to stop the infection spreading further. The Doctor tells Hex that someone has tampered with Sistermatic to stop it from asking difficult questions so he asks his friend to find out more about what really happened while he visits Professor Shimura.

Khan passes Ace a pulse-beam emitter and says it can punch a hole through anything organic with one blast. Unfortunately it would take a dozen shots to stop just one Dalek and they're unlikely to get that many chances. Stokes is dismissive of Khan's eagerness to engage the Daleks, but Ace has some ideas that might help increase the weapon's efficiency. Stokes tells them to be quick as the Daleks have activated the external proximity alert.

Outside the facility, the Daleks detect a laser defence grid around the base, but they were prepared for this and continue with the next stage of their attack strategy. They contact the people inside and offer them safe passage from this world if they surrender. Ace is baffled by the change in the Daleks' normal policy of exterminating first and asking questions later, but Stokes tells her their latest reports suggest the Dalek need expendable labour. Khan is furious as Stokes had earlier refused to look for survivors from their fleet on the ground that Daleks never take prisoners. She accuses her commander of abandoning her post and tells her she's a coward. Stokes orders Khan to collect the walking wounded from the infirmary and form them into teams, then she tells Ace to go with her and make sure she gets it right. Ace makes it clear to Stokes that she agrees with Khan and tells the Lieutenant it's about time she started leading her troops. Stokes admits that she can't bear to face the Daleks again and she deactivates the defence grid. Stokes is resigned to her fate and thinks Ace is only prolonging her agony by resisting. She tells Ace she's seen the Daleks slaughter women and children indiscriminately and whole families killed en masse. She believes a quick death will be the best thing for all of them. She asks Ace to forgive her, then she runs away.

The Doctor is busy trying (and failing) to decrypt the pass code to enter Shimura's laboratory when Ace calls him on the communicator. She tells him what Stokes has done and he asks her to get the defences back up, but she has little success. The Doctor admits that he's struggling to find the pattern to the pass code as he knows nothing about how the Professor's mind works. She reminds him of the tune that Shimura was singing earlier and the Doctor recognises the code as integer-rotation and repeats it using the entry buttons. The door opens and he enters the laboratory.

The Dalek assault force leader informs the Black Dalek that the humans have deactivated their defence grid. The Black Dalek says he'll join the others on the surface once the facility is secure. The first wave of Daleks advance towards the building. Ace watches them on the scanner and chooses her moment carefully. Once they reach the perimeter, she switches the laser grid back on and the Daleks at the front are destroyed. The assault force leader quickly alerts the other Daleks and they retreat to safety.

As the Doctor explores the laboratory, Hex contacts him and says he's had no luck cracking the code to the isolation room. The Doctor gives Hex the code he used himself earlier and then reveals that he's discovered some precision engineered cocoons. They're larger than a man and made of soft silk, but spun from metal. Suddenly Shimura confronts him and tells him he's the first person, apart from himself, to see the cocoons.

Hex is about to enter the isolation room when Ace contacts him and tells him she's just destroyed a dozen Daleks with the defence grid. Hex goes inside and examines the patients. He realises straight away that their wounds were not caused by an infection - in fact, it looks like something has eaten its way out from inside their bodies. Then one of the patients wakes up and pleads with Hex to kill him. Hex refuses, just as Sistermatic enters and demands to know what he's doing here. Hex confronts the droid and points out that the wounds are actually bite marks, which Sistermatic is unable to explain.

The Daleks outside the facility focus their weapons on the laser grid emplacements and open fire. The defence system is swiftly disabled and the assault force advances.

Ace warns the Doctor that the Daleks will soon be inside the base. The Doctor informs Shimura, but the Professor isn't surprised as the grid was only designed to keep out the Piranha Locusts. The Doctor tells him he encountered some of the creatures earlier and they were five times their normal size and that kind of growth would normally take a thousand generations of evolution. He accuses Shimura of interfering with their development and realises it would have required access to gestation chambers and equipment prohibited throughout the galaxy by ethic research laws. The Doctor says he understands the need to break the rules sometimes and Shimura says it's refreshing to meet someone who shares his attitudes. He admits that enlarging the Piranha Locusts was the first experiment he conducted when he arrived here, but the creatures inside the cocoons are their next stage of development and are the results of a lifetime's work. Shimura calls them Kiseibyaa, the saviour of mankind, and orders the Doctor to kneel before them.

Khan reports that the Daleks have entered the north-east entrance and have already killed four members of the Valkyrie Unit. The rest of them are fighting a rear-guard action to try to slow the Daleks down, but it's proving fruitless and they're losing more people all the time. Ace reveals that Stokes has deserted her post. She calls up a schematic of the base and tells Khan there's a narrow corridor close by that should be easy to blockade. She suggests they send half the team to pack explosives around the walls, but Khan tells her they're close to the infirmary and there are patients still inside. Ace realises Hex will be trapped there too, but before she can react she hears a scream and communication with Khan is lost.

Ace calls Hex in the infirmary where they can already hear the sound of fighting outside. She tells him to get out, but Hex refuses to abandon the patients. Sistermatic offers to stay behind to look after the others, but it's too late. Seconds later the doors to the infirmary are blown open and a group of Daleks advance into the room.

Shimura tells the Doctor he's about to witness the birth of a new species. Cracks start to appear in the cocoons and the Professor urges the creatures inside to push their way out, like a mother encouraging a new born baby. Moments later, the first of the Kiseibyaa emerges.

The Daleks confront Hex and Sistermatic and detect the presence of seven other humans nearby. Hex assures them the patients are too sick to pose any threat to them, but the Daleks regard all humans as their enemy. Hex asks them to show mercy, but the Daleks place no value on that word.

Ace warns the Doctor that the Daleks are swarming into the base, but he's too busy to respond. She quickly tells him she's found something hidden within the building schematics - it's a homing beacon and the signal is being beamed into deep space. This means the Daleks didn't follow the Valkyrie Unit here, they were summoned by someone already in the facility. Shimura is overcome by the beauty of the creature he's created, but the Doctor is horrified and describes it as a living car crash. The Kiseibyaa tries to speak to them and Shimura explains that he often spoke to the creatures inside their metal wombs. Then the Professor steps back and tells the monster that the Doctor can be its first meal. Sensing the flesh before it, the Kiseibyaa takes its first steps towards him.

The Daleks in the infirmary suddenly come under attack from Sergeant Khan and her remaining troopers. Khan orders Hex to leave and although he's still reluctant to leave the patients, he knows he has no choice. The Daleks open fire, but Hex is able to escape in the confusion and eventually re-joins Khan and the others in a corridor not far away. She hands him a pulse-beam emitter and shows him how to use it, but he tells her he's a nurse and she should leave this sort of thing to Ace. He insists on returning to the infirmary and refuses to listen to Khan's objections.but when they hear the sound of the patients being exterminated, he finally realises it's too late to help them. Hex is devastated and assumes this is the atrocity the Doctor was referring to earlier. Khan and the troopers race off and Hex reluctantly follows them.

The Doctor tries to reason with the advancing Kiseibyaa, but it has no effect and he yells to Shimura to call off the creature. The Professor ignores him and encourages the Kiseibyaa to attack, but the creature tells him it hungers, but not for flesh. The Doctor realises Shimura knew all along that this would happen and the Professor laughs at his own little joke. He explains that his experiment has been a success and reveals that the Kiseibyaa are a species that feeds on metal. The creature is hungry so the Doctor suggests it starts with the Professor's genetic-splicing equipment. Shimura protests, but the Kiseibyaa is clearly ravenous and he realises it would be dangerous to interfere with its first meal. Soon the others of its kind will emerge and none of them have eaten since their feast of flesh before they were cocooned.

The horrible truth is slowly dawning on the Doctor - the dozen or so human bodies they found outside in the forest had been torn apart and eaten alive and now he knows what happened. Shimura admits that things got out of hand and there never was an evacuation of the base. He also reveals that the Piranha Locusts are the Kiseibyaa's larvae stage, during which time they consume many times their own weight in raw meat. Even at this stage of their development, they remain voracious and he's surprised by how much of the equipment in his laboratory this single specimen is devouring. It disturbs Shimura that he's not able to control the creature as easily as he'd expected and he's unable to stop it eating everything in sight - yet the Kiseibyaa still demands more metal. Shimura promises to bring some soon and is shocked when the huge creature lashes out and strikes him. The Doctor recommends they leave the laboratory immediately.

Ace contacts Hex and is relieved to discover he got out of the infirmary safely. She needs to leave the command centre as the Daleks are starting to close in, so Khan suggests they find somewhere to make a stand. Ace checks the schematics and decides the kitchens will be the easiest place to defend. It'll take the Daleks ten or twelve minutes to reach that part of the base so everyone agrees to meet there and they'll seal the doors in eight minutes time. After that, nobody will get in or out.

The Daleks realise their enemies are retreating and prepare to exterminate anyone caught hiding. The assault leader contacts the Black Dalek and reports that the research facility is now under their control. The Black Daleks orders them to search the computer to discover whether it contains any information of value. Suddenly Lieutenant Stokes emerges from cover and confronts them. The assault leader prepares to take her prisoner, but she's been in one of their slave camps before and has no intention of going back. She insists that they kill her now, but she is unarmed and they do not consider her to be a threat. She becomes hysterical and pleads with them to exterminate her.

Shimura is still in shock after the Kiseibyaa attacked him. The Doctor seals the laboratory door and is relived to discover it's made from a toughened non-metal substance. It won't stop the creatures forever, but it should be long enough to give him time to review Shimura's notes. They're dealing with a new and unknown species so the Doctor wants to assess what its next move will be. Shimura says he created them to save humanity, but the Doctor says they couldn't care less about that. It's an intelligent predatory animal, but all it wants to do is feed and breed. They head off for the Professor's private quarters.

Khan's officers fall back, but one of them, Domingo, is killed before she can join the others. Khan is furious with Stokes and blames her for not doing her duty, but Hex points out that they'd probably be dead already if she had. Suddenly the Daleks appear in the corridor behind them and Khan is nearly hit by one of their energy blasts. It causes her weapon to blow up and she screams in agony. She discovers she's been blinded and although Hex examines her and assures her she'll be alright, she knows he's lying. The Daleks order them to surrender, so Khan orders Hex to leave her and make his way to the kitchens, but he drags her to her feet and forces her to march to safety.

Ace arrives in the kitchen area and hears someone sobbing from behind the cooker. She orders the person to come out from where she's hiding and discovers that it's Stokes. The Lieutenant is in a bad way and cries hysterically that she deserves to die.

The Doctor and Shimura are nearly at the Professor's private quarters when they have to take cover to hide from a passing Dalek patrol. As they try to find an alternative route, the Doctor asks Shimura if he's ever wondered how the Daleks came to be. He explains that they were created by a scientist to break a deadlock in a war. He believed they would save his people but instead they exterminated them and his name became a byword for scientists gone mad. Shimura insists that his Kiseibyaa are nothing like the Daleks, but the Doctor says they will be if he doesn't try to correct their development. Shimura says he was inspired by the silk worms that his father kept as a hobby. As a child, he wondered what kind of cocoons they would spin if he tried feeding them something different, and this is why he grew the Ironweed forest outside. Unfortunately his funding ran out and he was forced to sell his talents to the military. The Doctor can't help thinking history is repeating itself.

Ace tries to reassure Stokes that the others will be joining them soon, but the Lieutenant is convinced they've all been killed or worse, taken prisoner. Hex calls Ace and tells her he's not sure where they are and there are Daleks everywhere. They hear the sound of Dalek fire and then the communication is cut off. Ace has no choice but to go and find him, but Stokes becomes hysterical at the thought that the Daleks will kill her.just like they killed her sister.

Once they arrive at Shimura's quarters, the Professor can't wait to show his research notes to the Doctor. They show how he combined DNA markers from the silk worm and the Piranha Locusts to create a species that could spin gossamer threads of metal, as subtle as silk yet stronger than any known alloy. To Shimura's surprise, the Doctor uses his computer to tap out a message to the Daleks, telling them where they are. He wonders why Shimura appears so horrified by the idea, considering it was he who summoned them to Bliss in the first place. Shimura admits this is true. When word reached them that the Daleks were attacking refugee fleets in this sector, the other scientists at the base wanted to evacuate, but he saw it more as an opportunity. He believes the Daleks are successful as a species because unlike most predators, who are themselves prey for other species, they were manufactured to have no natural enemy. He therefore engineered a new species, the Kiseibyaa, who would prey on the Daleks. The military had been looking for a new weapon that would hold back the Daleks and stop the war, so he created one monster to stop another. The Doctor believes Shimura is a scientific genius, but says he wasted his brilliance on an act of madness that will endanger every lifeform in the galaxy.

Hex is struggling to carry Khan when they hear the sound of a Dalek nearby. After a short commotion, which ends with an explosion, Ace appears at the end of the corridor and runs over to join them. She's shocked by Khan's condition, and Khan herself is still dubious about Hex's claim that she can be fixed up. They're not far from the kitchens, so Ace takes the lead and the others follow close behind her. It looks like they, and Stokes, are the only survivors and they've lost all radio contact with the Doctor. They arrive at the kitchen just as another Dalek patrol turns the corner and opens fire. Ace fires back while Hex and Khan race inside, then she joins them and seals the doors with a blast from her pulse emitter. Hex starts attending to Khan's injuries while Ace looks for Stokes. She finds the Lieutenant curled up in a ball on the floor, humming to herself like a child.

The Dalek spaceship descends onto the surface of Bliss and the Black Dalek disembarks. He's informed that their enemy's computers reveal that the humans have been developing a new anti-Dalek weapon inside the facility. He's also told that several survivors have barricaded themselves into a secure area, but Daleks with thermo-cutting devices have been sent to extract them. He then orders the assault force leader to escort him to the weapons laboratory.

Shimura is surprised by the Doctor's dismissive attitude towards his work, but the Doctor has by now realised the Professor must have injected his own colleagues with Kiseibyaa eggs to turn them into human incubators. Shimura argues that it was a necessary evil as the Daleks were expected to control the whole sector of space within days and his fellow scientists would have died anyway, and for nothing. The Doctor insists that their lives were not his to sacrifice, but Shimura says that if he'd been given enough time he could have purged the gene from his creations that made them want to feast on human flesh. He was forced to take a short cut that turned one of the most beautiful planets into a factory of death. Shimura reminds the Doctor that the Kiseibyaa were created to save mankind, but the Doctor points out that in the Japanese language, his creations' name means "parasite saviour". Shimura claims this was just a private joke, but the Doctor believes he knew all along what he was doing.

In the kitchens, Khan can hear the Daleks starting to burn their way through the door. She tries to see the positive side and notes that she probably won't live long enough to see them anyway, and although Hex tries to assure her she'll be alright, he eventually admits that she's probably right. Stokes appears to have had a complete mental breakdown and Khan suspects she's retreated inside her own mind because she was unable to run away in person. Ace comforts the disturbed Lieutenant and asks her about the lullaby she's singing. Stokes tells her about her childhood when the Daleks invaded her home world. Her sister Rebecca kept her safe as long as she could, but one day she went outside and the Daleks followed her back to their hiding place. Her sister swore she'd never end up in a labour camp, so she tried to run but the Daleks killed her. On the other side of the room, Khan calls out and taunts Stokes for trying to play the martyr. She's angry that she's having to spend her last few minutes listening to her commander's life story. She reminds Stokes that everybody has lost somebody they loved to the Daleks and they've all suffered, but that doesn't give her the excuse to run away. She again accuses Stokes of leaving the refugees behind to die and says the only thing she achieved was leading the Daleks right back here. Ace corrects her and says the Daleks didn't follow the Valkyries here - they were summoned by someone else. Stokes realises that what's happened here isn't her fault. Hex tries to get Khan to acknowledge that she was wrong, but it's too late. The strain has proven to be too much for her and she's dead. Ace tries to comfort him, but he knows this is something he'll never get used to.

The Daleks discover the laboratory door controls have been deliberately overloaded. They start to work on de-coding the lock.unaware that their presence outside has been detected by the newly born Kiseibyaa. It turns to the other cocoons and urges its fellow creatures to fight their way out as their birth-feast is waiting. The laboratory door eventually slides open and the Daleks glide into the shadows.

Ace only has enough energy left in her weapon for about seven more shots, which is nowhere near enough to hold off the Daleks. Stokes takes the gun from her and says they can't afford to waste a single shot. Even though it wasn't her fault that the Daleks came here, she still knows she gave in to her fears and blames herself for the deaths of her officers, including Sergeant Khan. She thinks it's time she made the Daleks pay for what they've done and she decides to take the battle to them.

With the laboratory in darkness, the Daleks are forced to recalibrate their vision to infra-red. They register residual traces of increased temperate and soon discover the empty cocoons, but they're unable to identify the species. They start to search for the creatures that emerged and detect life signs on the ceiling. Seconds later, the Kiseibyaa drop down to the floor and attack the Daleks.

Stokes warns Ace and Hex to take cover, then she prepares to take on the Daleks as soon as they break through the doors. The Doctor manages to make contact with his companions, but his voice is distorted by magnetic interference and they can't make out anything more than a warning to beware something. Just then, the doors open and the Daleks swarm into the kitchen. Almost insane with fury, Stokes returns fire with her pulse-beam emitter.

The Daleks fight for their lives in the laboratory, but their enemies are moving too fast for them to lock on their weapons. They start to fire at will, but within seconds one of the Kiseibyaa has grabbed hold of the Black Dalek and begun consuming its casing. The other Daleks watch helplessly as their leader starts to malfunction, then they withdraw back into the corridor. The Black Dalek calls out, but to no avail.

Stokes proudly tells Ace and Hex they can come out from cover. Ace is genuinely impressed, but there are bound to be more Daleks on their way so they have to get away quickly. Ace thinks its odd the Dalek reinforcements aren't here already and guesses they must have been needed elsewhere. Stokes is filled with euphoria and can't wait any longer, so she races off down the corridor in search of more Daleks to kill. Hex tries to contact the Doctor again, but there's no reply. They remember that the communicators can also be used as tracking devices, so they set off towards the command centre where they can locate him.

The Doctor notices that the Kiseibyaa still haven't left the Professor's laboratory and Shimura assumes it's because it's the only home they've ever known. Just then the creatures become active again and head off for some more Dalek hunting. Their creator is overwhelmed by how awesome they are, but the Doctor is more interesting in seeing what's left of the laboratory. They soon find the remains of the Black Dalek, its outer casing ripped completely apart. The Doctor pities the creature, but the Black Dalek is still alive and tells him it needs no pity. The Doctor offers to help him, but then the firstborn Kiseibyaa emerges from the shadows and warns him to keep back. Shimura notes that its hide is a darker colour than the others, which means it's further along their life cycle. The Kiseibyaa tells them it's eaten and laid its eggs - inside the Black Dalek. When its children hatch, they'll have a ready food supply of flesh. The Doctor almost feels sorry for the Dalek and Shimura agrees that the process is unpleasant. The Doctor again warns him that the Kiseibyaa pose a great threat to mankind, but when the Kiseibyaa leaves to rejoin the others of its kind, Shimura races off after it.

The newborn Kiseibyaa encounter one of the Dalek groups and, ignoring the warnings, they attack the Daleks and begin ripping apart their metal casings. Elsewhere in the facility, the assault force leader learns of the attack and concludes that the humans have deployed a new species as a weapon against them. The Dalek forces are greatly outnumbered, but they cannot withdraw from the base until they've received instructions from the Black Dalek. At that precise moment, Ace and Hex turn a corner - having got hopelessly lost in the corridors - and find themselves confronted by the Dalek group.

The Black Dalek is in a very bad way and the Doctor is surprised it's still alive. The Dalek recognises him as their greatest enemy and the thought of exterminating him seems to invigorate it slightly, but it's too badly damaged to take any action. Its weapon has actually been eaten and the Doctor wonders how the Black Dalek feels about encountering a new species that regards them as nothing more than lunch.

To Hex's horror, Ace tries to bamboozle the Daleks by humming along to the tune she heard earlier. They order her to stop and Hex readily agrees with them, but Ace argues that if the Daleks had any intention of killing them, they'd have done it already. Hex becomes angry and says he's seen and heard more people die today than in the rest of his entire life and he's had enough. The Daleks threaten to interrogate Ace, but she tells them to kill her straight away as she has no intention of helping them. Hex finally snaps and agrees to tell the Daleks anything they want to know on one condition - that they promise to let them both live. Now it's Aces turn to be angry as Hex freely reveals that they came to this planet with the Doctor. The Daleks become alert and tell Hex that he and Ace have value to them after all. Once they've been interrogated, they will both become Dalek slaves.

The Black Dalek struggles to accept its fate, but the Doctor likens it to a fly in the hands of a wanton boy. Knowing it will soon be eaten by the larvae of the Kiseibyaa that have been laid in its body, it orders the Doctor to exterminate it by overloading its power cells. This will trigger an explosion large enough to destroy the entire facility, wiping out all the Kiseibyaa at the same time. The Doctor asks why he should help it, but the Black Dalek points out that these creatures are a threat to all species, not just the Daleks. The Doctor suggests placing a quarantine around the planet to prevent them endangering other worlds. He points out that the Daleks have devastated whole solar systems, subjugated thousands of species and spread fear and terror across the galaxy, so why should he spare the Daleks the same horrors they've inflicted on others? The Black Dalek pleads with him to show mercy and says it knows he places value on that word. The Doctor refuses to make a decision and leaves, forcing the Dalek to call out to him in desperation.

Elsewhere, another group of Daleks find themselves under attack from the Kiseibyaa. They fire at will, but the attack is too swift and the Daleks are forced to call for reinforcements to come to their aid. The assault leader guarding Ace and Hex orders his fellow Daleks to go to the assistance of the others. Ace tries to convince Hex that he's totally wrong if he thinks he can come to a deal with the Daleks, but Hex says the Doctor can look after himself and it's him and Ace that he's worried about. Ace reminds him what happened to Stokes and says she'd have been better off if the Daleks had exterminated her, but Hex doesn't really believe that. The Daleks soon realise the enemy forces are too strong for them and they prepare to withdraw back out onto the planet surface. However, all the escape routes have been blocked by the Kiseibyaa and they're forced to create a new way out. Suddenly a nearby door opens and one of the Kiseibyaa bursts through and attacks the Dalek assault leader. Within seconds the Dalek has been destroyed, and although Hex is sickened by what he's seen, Ace thinks it's no more than the Dalek deserved. The Doctor arrives and tells them they need to save Professor Shimura from himself as he's gone off in search of the first-born Kiseibyaa.

Shimura wanders down the empty corridors, singing the lullaby out loud until it attracts the Kiseibyaa he's been looking for. The creature recognises the tune and Shimura explains that he used to sing it to them to soothe their growing pains while they were cocooned. The Kiseibyaa also recognises Shimura and says all human flesh has its own unique stench. The Professor is disappointed and admits that he never realised how alien his creations were. He always believed he could control the creatures, but the Kiseibyaa tells him he was wrong. Shimura says he wants to be part of the next generation of Kiseibyaa and invites the creature to bury its eggs inside his body. The Kiseibyaa agrees and strides towards him. Seconds later, his screams fill the air.

The Doctor, Ace and Hex hear the screams and the Doctor asks his friends to stay behind to guard his back while he investigates. By the time he reaches the Kiseibyaa, he finds it standing astride Shimura's dead body, singing the lullaby to the eggs it's just laid inside him. The creature tells him Shimura offered his body as a means of incubating its young, and it suggests the Doctor might want to do the same. Instead, the Doctor tells it that the forest outside consists of trees with mercury for sap, which will be more than enough to sate the Kiseibyaa offspring's need for flesh. He says they could survive here for millennia, but the creature tells him survival is not enough and they hunger for much more. The Doctor recognises the nature of the Piranha Locusts, but says they can outgrow that if they wish. The Kiseibyaa refuses to listen and moves towards him, forcing him to race back down the corridor.

Lieutenant Stokes approaches the injured Black Dalek in the laboratory and reminds it that she once begged it to show mercy when it was attacking their refugee fleet, but it refused. She taunts it and revels in its defenceless state. The Black Dalek invites her to take her revenge by killing it, but eventually she lowers her weapon. She tells it she spent years in Dalek labour camps, watching everyone around her die. She had to fight other prisoners for scraps of food and she betrayed everything she believed in just to stay alive. Although she blames the Black Dalek for making her what she is today, she still finds it difficult to pull the trigger. The Doctor, Ace and Hex arrive and the Doctor tells her not to torture herself. Hex sees what's happened to the Dalek and turns away to be sick. The Dalek can feel the Kiseibyaa parasites moving inside it and knows that they'll hatch soon.

The Doctor plans to set the Black Dalek's power cells to self-destruct, knowing it will destroy the entire facility and the Kiseibyaa too. Ace and Hex point out that this would be genocide and Stokes reminds him that these creatures are the only ones that can totally defeat the Daleks. Ace realises that when the Doctor said one of the worst atrocities of the Dalek war would be committed here, this is what he was referring to. Hex likens it to America dropping the bomb to end the Second World War and wonders whether the cost is worth it. The Doctor tries to get his friends to understand - sometimes he has foreknowledge of events and sometimes he's part of events, but he knows what can happen if the wrong choices are made. He recalls being present when the Daleks were first born and admits that he had a chance to avert their creation, but at the time he believed wiping out an entire intelligent lifeform would make him no better than them. He knows he probably doesn't have the right to destroy the Kiseibyaa, but he also knows it's his duty and his place in history. If he doesn't make sure things unfold correctly right here and now, the Kiseibyaa will bring a terror to the galaxy that makes the Daleks pale in comparison.

The Kiseibyaa have now destroyed all but two of the Daleks in the facility. The Daleks order their vessel in orbit to leave to ensure they don't inadvertently become carriers for the Kiseibyaa - then they both destroy each other.

The Doctor is ready to trigger the power overload that will blow up the facility, but Stokes says she wants to do it. After everything that's happened, she's concluded that to destroy a monster you first have to become a monster. The Doctor disagrees with her reasoning, but eventually she persuades him to let her do it. Following his instructions, she connects two wires together and the countdown begins. The Doctor estimates they have just a few minutes, but it should be enough for them to get clear. As the others leave, the Black Dalek tells the Doctor that the ruthlessness he's shown today has been impressive. Just then, they hear the sound of the Kiseibyaa approaching and the Dalek delights in telling the Doctor that he will share his fate here. The Doctor realises he's going to have to stay behind in order to make sure the Kiseibyaa don't interfere with the countdown. Stokes steps forward and reminds him that she's a trained soldier and is better equipped to hold the creatures off. In any case, she doesn't want to live anymore.

Reluctantly the Doctor leaves and catches up with Ace and Hex further down the corridor. They race for their lives just as the Kiseibyaa appear and head for the laboratory. Stokes raises her pulse-beam emitter and opens fire. Several of the creatures are ripped apart, but then, to the Lieutenant's horror, the weapon stops working. The Black Dalek tells her it doesn't matter now as the time has arrived. Stokes mutters the name of her dead sister - and then the entire complex is destroyed in a huge explosion.