Working late at her florist shop, Rani’s mother Gita phones her daughter to explain that she has received an emergency order from an unnamed woman and will be working through the night. Soon there is a knock at the door and Gita discovers it is her client, who has come to hand over a cheque for the flowers. Gita looks at the name on the cheque and muses that it is an unusual one…Wormwood.

That same night, on Bannerman Road, Luke tosses and turns in his sleep, haunted by visions of the day he was born in the Bubbleshock factory. He awakens to see the woman who created him – Mrs Wormwood – standing at the end of his bed, calling his name. He hastily turns on the light but when he does so, she is gone.

The next morning Sarah Jane comforts her son, claiming what he witnessed was simply a nightmare, and that the real Mrs Wormwood did not know he had been called Luke, for she was destroyed before Sarah chose his name. Luke is still not convinced, claiming he has never dreamt before now.

There is a knock at the door and Sarah hurries to answer it. It is Rani, who is in a state of shock after discovering her mother has not returned home from work. Sarah and Luke hurry over to Rani’s house to ask Haresh about the incident but they learn little else, only that the shop floor was found covered in flowers – as if a struggle had taken place.

Sarah then takes Luke to Gita’s shop, where they are joined by Clyde. Sarah picks up an alien signal in the vicinity but the trace is too weak to identify which species. However, upon finding the cheque on Gita’s desk, Luke realises it is the Bane, the race that created him.

Hurriedly returning to Bannerman Road, Sarah speaks with Mr Smith, suspecting the cheque is a clue left by Mrs Wormwood for her to find. Sure enough, when Mr Smith scans the cheque, he discovers it is concealing an address, that of an abandoned warehouse. Sarah takes it as an invitation and vows to go to the warehouse. Rani, who has just arrived, demands to go as well, and Sarah agrees, although she tells Luke to stay behind in case Mrs Wormwood is after him. Clyde remains to look after his friend.

Soon Sarah and Rani arrive at the warehouse, whereupon Sarah recalls the last time she encountered the Bane, namely their plan to use the drink Bubbleshock to take over the Earth, and their attempt to overcome the 2% unaffected by the drink (of which Rani is a member) by creating Luke, a distillation of thousands of humans who visited the Bubbleshock factory.

Going inside the warehouse, the girls find Gita stood in the middle of one of the holding areas, seemingly in a trance. Rani rushes over to her as Mrs Wormwood arrives to confront Sarah, claiming that she has been watching Sarah and her friends for some time, hence her kidnap of Gita to get their attention. At last she reveals why she has called upon Sarah – she requires her help. She claims that Earth, and the rest of the galaxy, is under threat, and Sarah must help to avert disaster.

Soon two men enter and confront Mrs Wormwood, calling her an enemy of the Bane and ordering her to surrender or else be devoured. One of the men transforms into the true Bane form, a squid-like creature, but Mrs Wormwood uses her ring to distract it, allowing her, Sarah, Rani, and the transfixed Gita to escape.

As they flee down a corridor, the Bane uses one of its tentacles to grab Sarah’s leg, and it drags her back towards the holding area. Reluctantly Mrs Wormwood goes after her and manages to free her, and then the two women return to escape with Rani and Gita. Mrs Wormwood explains that after the Bane Mother was destroyed by Sarah in the Bubbleshock factory, she was thrown out of the Bane Kindred and is now hunted. However, she is fighting back; not to save her own life but to stop the Bane conquering the galaxy.

Returning to Bannerman Road, Mrs Wormwood releases Gita from her trance, and Sarah convinces her she had been sleepwalking. Gita appears to recognise Mrs Wormwood but she claims they have never met.

As Rani leads Gita home, Sarah takes Mrs Wormwood into her house, where Luke and Clyde are waiting. They all go up to the attic, where Sarah calls upon Mr Smith and tells him to place Mrs Wormwood in a containment vortex if she causes trouble. Rani arrives; in time to hear Mrs Wormwood talk about the legend of Horath and the Dark Empire.

Mr Smith explains to Sarah and the others that the Dark Empire was a period of tyrannical rule by a reportedly-immortal entity named Horath. Mrs Wormwood explains that Horath was defeated but not killed, so his body and consciousness were separated and hidden at opposite ends of the galaxy. The Bane have found his consciousness (which is being carried by a mercenary agent) and now they want the body, which is hidden on Earth. As revenge for being outcast, Mrs Wormwood intends to thwart their plans.

Sarah asks where Horath’s body is but Mrs Wormwood does not know. Mr Smith explains that the Tunguska Skull, a relic charting the legend of Horath and his demise, will be able to help them, but it is kept in the Black Archive, the repository of all alien artefacts deemed highly dangerous, an archive owned by UNIT.

Ordering Mr Smith to contain Mrs Wormwood, Sarah and her friends hurry downstairs, but again Sarah insists Luke must remain behind in case UNIT grow suspicious of her actions and come after him. She tells Clyde to look after him and orders them not to go near Mrs Wormwood. They agree and, after hugging Luke goodbye, she and Rani prepare to leave. However, before they go to the black archive, Sarah claims she must visit an old friend first.

Brigadier Sir Alistair Gordon Lethbridge-Stewart, in his home in the country, is visited by Major Kilburne, who has arrived to debrief him on his mission to Peru. However, the Brigadier is not interested, claiming in his retirement he is beyond missions, debriefings and the fools-errands he is sent on by UNIT as the official envoy. Major Kilburne protests, stating his official duty, but the Brigadier is again uninterested, instead claiming that UNIT has lost the benefit of common sense.

Kilburne claims UNIT has adapted to a more hostile age, but the Brigadier turns on him, claiming his encounters with Daleks, Cybermen, Autons and Zygons far outweigh Major Kilburne’s operations.

At that moment Sarah and Rani arrive, and the Brigadier welcomes them, as does Major Kilburne, who has read many of Sarah’s UNIT files. He bids the group farewell and leaves. Sarah, now in the Brigadier’s confidence, tells him of her intentions – she needs to break into the black archive. The Brigadier listens and when she reveals her plans, can only reply;

“Oh.”

Back in Bannerman road, Clyde and Luke talk about parents, and how important they are. Luke claims that for this reason he must see Mrs Wormwood, and reluctantly Clyde agrees. Luke hurries to the attic, where Mrs Wormwood, almost tenderly, speaks to him about his life, and how difficult it is for him to fit in, to exist alone in the universe without kindred, just like her.

The Brigadier takes Sarah and Rani to the Black Archive, a massive security facility. Sarah and Rani hide in the back of the car so as not to arouse suspicion, and the Brigadier drives through the gates. He checks the coast is clear and finds Major Kilburne standing around. He manages to move him on, claiming that he is visiting the archive to research for his memoirs. Kilburne leaves and Sarah and Rani get out of the car. The Brigadier hands them a key card and stands watch as they hurry inside.

The archive is a massive warehouse of alien artefacts boxed up and stored away. Sarah uses a database to find the location of the Tunguska Scroll, and she and Rani hurry away to find it.

As Major Kilburne and the security guards monitor the CCTV cameras, the girls at last find the vault containing the scroll, and with the help of the Sonic Lipstick manage to retrieve it.

On Bannerman Road Clyde answers a call at the door, and discovers that one of the Bane is waiting for him. He calls up to Luke in the attic and Mrs Wormwood, fearing for her life, begs Luke to help her:

“Luke, I am your mother!”

In the Black Archive, Sarah and Rani are discovered. An alarm sounds and the two girls hurry back to the Brigadier, the scroll still in their possession. Major Kilburne and his men are soon on their tail.

Freeing Mrs Wormwood, Luke and Clyde flee into the back garden, but the Bane is hot on their heels. Mrs Wormwood tries to fend the creature off with her ring, really a Sonic Disruptor, but its power has been drained. The creature corners her as Luke and Clyde escape to the front of the house, where two more Bane are waiting for them.

Sarah and Rani return to the Brigadier’s car, where Major Kilburne and his men are waiting for them, guns aimed. Sarah zaps their earpieces with her Sonic Lipstick, giving her and her friends a chance to escape. They get in the car and the Brigadier speeds away, narrowly avoiding the closing gates.

In Sarah Jane’s garden, Mrs Wormwood cowers from the Bane creature, when it is suddenly blown to pieces…by Sontaran Commander Kaagh. Mrs Wormwood greets him and asks if he has the Consciousness of Horath. He claims it is hidden, but it is still safe. Mrs Wormwood is delighted.

Trapped by two Bane creatures, Clyde and Luke try to fend them off with a hose, but help is on its way. After Kaagh recharges her Sonic Disruptor, Mrs Wormwood steps out and uses it on the Bane, causing them to explode and splatter their remains all over Luke and Clyde.

Soon Sarah, Rani and the Brigadier arrive, claiming that they have the scroll but must make tracks before UNIT trace them back to Bannerman Road. Unaware that Kaagh is watching them, Sarah and her friends hurry Mrs Wormwood to Gita’s shop, where they should be safe. Whilst Rani and Clyde make tea, Sarah thanks Mrs Wormwood for saving her young friends and asks her if she has any children, but she claims she does not.

After a bit of verbal sparring Sarah claims that the containment field created by Mr Smith would have been powered by any weapons that are present, and sure enough Mrs Wormwood reveals her Sonic Disruptor. Sarah quickly prevents her from using it by aiming her Sonic Lipstick at her.

To break the stalemate Mrs Wormwood calls out to Kaagh, who enters with Rani and Clyde in tow. After snatching the Lipstick and crushing it underfoot, Kaagh aims his gun at Sarah and threatens to kill her in order to wreak revenge for their last encounter. Mrs Wormwood reveals that Kaagh was working as a mercenary agent for the Bane, and was carrying the consciousness of Horath when he found her stranded on a distant planet. The two made a pact to find Horath and use his power to gain their revenge.

Reluctantly Sarah hands over the Tunguska Scroll and Mrs Wormwood and Kaagh prepare to leave, but not before Luke has bargained his friend’s lives by agreeing to go with Mrs Wormwood, who promises Sarah her son will remain safe as long as she does not meddle any further. Turning to leave, Mrs Wormwood blasts Sarah, Rani, Clyde and the Brigadier with her Sonic Disruptor.

Back on Bannerman Road, Gita knocks at Sarah’s door when she hears something moving in the garden. She finds Major Kilburne lurking around, claiming to be a relative of Sarah. She takes him back to her house for tea, where he claims Gita and Haresh must not contact Sarah and tell her of his arrival, for he wishes to surprise her.

Arriving back at the disused warehouse, Mrs Wormwood looks after Luke whilst Kaagh checks the perimeter for security. Mrs Wormwood turns to Luke and laments that after living on Earth the boy she created has become stunted – he was built for so much more. Luke objects, claiming he enjoys his life on Earth. He tells Wormwood and Kaagh he will not assist them, but Kaagh tells him he already has, for Sarah will not interfere whilst he is under threat. Kaagh then takes Luke and Mrs Wormwood to his space pod, where he reveals the conscience of Horath, a crystal that is placed inside the Tunguska Scroll.

Back in Gita’s shop, Sarah awakens and vows to find Luke, claiming Mrs Wormwood is sorely mistaken about the nature of motherhood if she believes she will not try to rescue her son. Luke meanwhile, immediately demonstrates his intellect by correctly guessing the Scroll and the crystal combined will create a tracking device that will lead Mrs Wormwood and Kaagh to the body of Horath. Again Mrs Wormwood remarks upon his superior abilities, before turning her attentions to the Scroll.

She explains that Horath was not a living creature; he was a super computer with the ability to command the physical nature of the universe. She looks upon Luke again and claims he is her only kindred, offering him great power if he agrees to be her prince. Revealing she has no real intention of sharing power with Kaagh, Mrs Wormwood tries to tempt her would-be son by handing over the scroll, which he immediately runs off with as an act of defiance. Kaagh is soon on his tail and a chase ensues throughout the disused facility, until eventually Kaagh tracks Luke down and holds him at gunpoint. He is about to shoot when Mrs Wormwood – reverting to true Bane form – intervenes, wanting Luke to be kept alive. She blasts him with the Sonic Disruptor as a warning then takes the retrieves Scroll, claiming their journey is almost over.

The Brigadier, Clyde, Rani and Sarah return to Bannerman Road, hoping to use Mr Smith to trace Luke. As the only person not recognisable to UNIT, whom the group fears may be hiding nearby, Clyde sneaks towards Sarah’s house. Finding nothing, he calls Sarah and gives the all clear, unaware something is watching him from the garden. He walks through the gate as the others approach the house.

Discovering Clyde to be missing, they step inside and find him being held by Major Kilburne, who aims a gun at them and orders them to hand over the Tunguska Scroll. Revealing it has been taken; Sarah watches as Clyde kicks the gun from Kilburne’s hand and wrestles him onto the sofa. The Brigadier takes Kilburne’s gun and aims his walking stick at him, Rani standing by his side. Clyde and Sarah hurry upstairs, where Sarah asks Mr Smith to use energy traces she has measured from the scroll to trace Luke.

Downstairs Major Kilburne turns on the Brigadier and Rani telling them that the home world security he seeks to protect is that of his own world, not theirs…

Upstairs, Mr Smith traces the scroll to a village called Whitebarrow, the site of a stone circle where Horath is buried.

Back in the living room Major Kilburne reverts to his true Bane form, but before he can strike the Brigadier reveals a secret weapon – a dart contained in his hollowed-out cane. He shoots the Bane creature between the eyes, just as Sarah and Clyde return downstairs. Together they all hurry outside, where Sarah asks Gita for use of her van. She hands over the keys and prepares to leave with Rani and Clyde, whilst the Brigadier remains behind to look after the incapacitated Major. He wishes the others luck, and they depart.

Meanwhile, Mrs Wormwood, Kaagh and Luke arrive at the stone circle in the Brigadier’s car, but find it is protected by a force field that stops non-humans from entering. Mrs Wormwood realises those who buried Horath must have known humans would never need to utilise the circle’s true secrets, so allowed them access but refused it to any aliens – who would be aware of the Scroll and the power it could yield.

As Sarah, Rani and Clyde speed towards the circle, Mrs Wormwood orders Luke to enter the stone circle with the scroll and place the Scroll in the centre stone. Reluctantly he does so, and immediately tendrils of light spread to each of the surrounding stones. The ground begins to shake, almost throwing Sarah and the others off the road, but they manage to power on as the ground within the circle gives way to reveal a portal to another dimension – the resting place of Horath.

As Mrs Wormwood laments her new life with Luke, Kaagh realises she intends to double cross him, but before he can take action she blasts him to the floor with her Sonic Disruptor, and he drops his gun. Once more she turns to Luke and offers him the power of Horath, but when Sarah arrives he runs to join her instead.

Again she tries to tempt him but he defies her, and Sarah rejoices that she is his true mother, for it is she who has cared for him, not Mrs Wormwood. She tells her nemesis that she does not truly understand love, she is a monster. Mrs Wormwood agrees, claiming she rejoices in accepting her own nature.

Clyde runs over to Kaagh, who is still on the ground. He asks Clyde to give him his gun, in order to kill Mrs Wormwood, but Clyde has learned a lesson from Sarah Jane, guns do not make things better. He refuses, and so Kaagh gets to his feet and, hoping to achieve some honour at last, charges at Mrs Wormwood and plunges her into the portal. A massive burst of energy marks their passing and Sarah, Luke, Clyde and Rani look on as the portal closes. Using a spare Sonic Lipstick she has been keeping safe, Sarah blasts the Scroll to dust, thus sealing the portal forever.

Soon Sarah, Luke, Rani and Clyde return to Bannerman Road to bid farewell to the Brigadier. Once he has departed, and after the night sky has fallen on London, the group of friends stand up in the attic and gaze at the stars.