Strange events are occurring in the village of Culverton, where postmaster Max Bishop and vicar Stephen Darnell are preparing to organise the summer fete. No sooner has the Ministry of Defense announced that the local aerodrome is to be decommissioned and sold to a private company, Legion International, than the former base commander vanishes without a trace. Legion's representative, a woman named Bliss, promises that her company will restore the neglected aerodrome and convert it into an international airfield, but Alec Whistler, a former Spitfire pilot, objected to the sale of the aerodrome and is suspicious of the newcomers' motives. Odd flashes of lightning are seen in the clear blue summer sky, and black lorries carrying strange torpedo-like cargoes are driven through the village at all hours. When teenager Noah Bishop is nearly run down by one of the blankly staring drivers, Captain McGarrigle of Legion is more concerned with getting the lorry back on the road than in seeing whether Noah is all right. Now convinced that McGarrigle and the private army of Legion security guards are up to no good, Whistler calls his old friend Alistair Lethbridge-Stewart and asks him to investigate. He is unaware that he already possesses something of great interest to Bliss and her men -- a lucky jade talisman he picked out of the ruins of the local pub after it was bombed in World War Two.

Charles Cochrane, the Secretary of Defense, is contacted by a mysterious blackmailer with photographs of Cochrane in a compromising position. Following the blackmailer's orders, Cochrane places the Culverton aerodrome out of bounds, and then meets secretly with the blackmailer to pay him off -- only to be attacked by something inhuman. Jocelyn Strangeways, the Chief of Staff, is also attacked by a scuttling, crab-like creature in his own home, and soon, every government official with authority over the Culverton aerodrome has been compromised. Meanwhile, in Culverton, Constable John Trickett visits the aerodrome himself to investigate the villagers' complaints about Legion, but Bliss assures him that someone will soon be coming to explain it all. Whistler and Noah Bishop, unwilling to wait for help to arrive, break into the aerodrome to investigate personally, but Whistler realizes too late that he's forgotten to bring his lucky talisman. In the grounds of the aerodrome, Legion's employees are unloading the lorries' cargo, but despite the nearly pitch darkness Whistler and Noah are spotted and are forced to flee. Whistler is captured and taken to Bliss, who tortures him, demanding to know the location of the "ninth key". Noah, meanwhile, cuts through the marsh to avoid the pursuing troops, but is attacked by something monstrous lurking beneath the swamp...

The Doctor returns to UNIT HQ following a gruelling vacation in which he assisted in the overthrow of the tyrant Gogon of Xanthos. The Brigadier, trying not to show his relief at his old friend's return, tells him of Whistler's call and the fact that his inquiries into Legion International have been blocked by the highest levels of the British government. The Doctor agrees to investigate, but as he and Jo set off, he admits to Jo that he is concerned that he has been institutionalised by his time on Earth. Upon arriving, the Doctor and Jo learn that Whistler has gone missing and have an unpleasant encounter with Captain McGarrigle, who appears to be searching for something near Whistler's home. While Jo remains with Whistler's housekeeper Mrs Toovey, the Doctor breaks into the aerodrome to investigate further, but he is spotted and chased into a wind tunnel by a silent, black-uniformed trooper. As they fight, a second, vertical wind tunnel is activated, and the trooper loses his balance and is shredded by the jet blades before the Doctor can deactivate it.

The Doctor returns to Whistler's home just in time to drive off McGarrigle, whom Jo had caught breaking in. Ted Bishop then calls his old friend Mrs Toovey for help, having found Noah unconscious and bleeding near the aerodrome. Ted's brother Max finds the emergency number inexplicably out of service, and attempts to report the incident to Constable Trickett -- but Trickett has returned from the aerodrome a changed man, and soon the same has happened to Max. The Doctor reports to the Brigadier, who manages to get grudging Ministry approval for a brief tour of inspection. Benton and Yates accompany the Doctor and the Brigadier to the aerodrome, but they learn little -- although the Doctor does manage to trip Bliss up in a couple of lies, thus proving to his satisfaction that Legion International is up to no good. He and his friends depart, unsatisfied and unaware that Whistler is attempting to escape, mere metres away. Whistler gets as far as the fence before being recaptured, but while trying to defend himself he punches McGarrigle in the throat -- and the crab-like thing inside McGarrigle is forced to find a new home for itself. More of the creatures are finding homes in the village, or rather in the villagers, and Trickett's terrified wife and daughter are on the verge of fleeing their home when Trickett arrives with an inspector from Scotland Yard -- who introduces himself as the Master...

The Doctor, Jo and the Brigadier return to the Bishops' home, where Noah is still in a state of shock after his narrow escape from the monster in the marsh. Unable to convince Ministry officials of the possible threat, the Doctor and the Brigadier attempt to return to the aerodrome, only to be forcibly turned away by the security guards. The Doctor theorizes that, if Legion International is indeed a front for an alien invasion of Earth, the vertical wind tunnel may be a means of simulating and preparing for their arrival. Meanwhile, Jo, feeling neglected, decides to investigate Noah's story, and Noah accompanies her, determined to find out what's happening. Upon arriving in the marshlands they see Legion's guards burying the black coffin-like objects which the lorries had been carrying, and as soon as the troops have gone, Jo and Noah dig up one of the objects -- to find a paralysed Mrs Toovey with a crab-like alien embryo clamped upon her face. The monster surfaces from beneath the marsh again and attacks, but the Brigadier and the Doctor arrive just in time, and the Doctor uses Bessie's headlights to blind the creature while he and his friends escape.

The Master, still in his guise of a Scotland Yard inspector, opens the fete and advises the villagers to check out the Legion International booth. He privately assures Bliss that he can deal with the UNIT team, but she advises him to be sure of success; if he fails her, he will be swept aside with the other inhabitants of this planet when the last of the Gaderene arrive. The Master and Bliss return to the aerodrome, where Whistler's interrogation has come to a halt; the embryo which escaped from McGarrigle has had to implant itself in Whistler to survive, and the process has subsumed Whistler's human personality, rendering him useless for interrogation. Bliss has no choice but to expel the embryo from Whistler, although this means its death; the Master then hypnotises Whistler, reads his mind and learns the location of the ninth key. Meanwhile, the villagers who enter the Legion International booth are treated to a realistic simulation of an airplane flight, complete with an emergency crash landing in which oxygen masks are deployed -- but instead of oxygen, the masks dispense Gaderene embryos which embed themselves within the terrified villagers' throats. By the time the Doctor and his friends return to Whistler's cottage, the village is deserted...

The Brigadier contacts the Ministry once again, and learns that most of the officials who were blocking his investigation have disappeared; the Master has brought them all Culverton to await the arrival of the adult Gaderene invasion force. With this obstacle removed, however, the Brigadier is able to summon Benton and Yates back to the village with a contingent of UNIT troops, but the Gaderene sense their approach. At the Master's suggestion, Bliss sends the implanted villagers to hold them off. The Doctor, meanwhile, tries to find out why McGarrigle risked breaking into Whistler's cottage earlier, and in the restored Spitfire out back he finds Whistler's lucky talisman -- a vital component of an alien matter transmitter which must have arrived on Earth decades previously. Without this component to stabilise the transmission, only the alien embryos can travel safely to Earth -- but with the final key in place, the adult Gaderene can survive passage through the vortex and the invasion can begin in earnest.

The villagers attack Whistler's cottage, but while the Brigadier and some of his men get past and prepare to attack the aerodrome, Benton and his remaining troops are faced with a moral quandary -- if they open fire on the enemy, they will be killing innocent people forced to act against their will by alien parasites. While Benton and his men try to hold back the villagers without killing them, the Doctor synthesizes nitrous oxide from the nitrates in Mrs Toovey's fertiliser and the iron filings amongst Whistler's belongings. He thus puts the attackers safely out of commission, but when Noah tries to see if his father and uncle are all right the Master arrives and captures him. The Master then demands that the Doctor hand over the jade key, the last component the Gaderene require for their invasion to succeed. The alien which occupies the human form of Bliss arrived on Earth with another of her kind during World War Two, but the transfer was unstable, and while she was stranded on Earth her kinsman was horribly mutated and was forced into hibernation in the marsh. The Master has since thoughtfully put Bliss back in contact with her people, and helped to arrange for the Gaderene to evacuate their dying homeworld and conquer the Earth. As the Doctor had suspected, only the embryos can survive transfer to Earth without the ninth key in place -- and unless the Doctor hands it over, the Master will kill Noah. The Doctor has no choice but to do so, and the Master departs to prepare for the invasion, while the villagers begin to recover and renew their assault on the UNIT troops.

The Brigadier and his men arrive at the aerodrome, only to find more villagers blocking their way -- and the Brigadier has no choice but to order his men to drive straight through the blockade, knowing that this will mean killing the innocent townspeople and children. Fortunately, the Doctor gets Whistler's Spitfire started and flies it to the aerodrome to stop the Master, and as he passes the blockade he drops more nitrous oxide jars, disabling the villagers and allowing the Brigadier and his men to pass through safely. Bliss detects the disruption, however, and uses her telepathic link to contact the monster beneath the marsh and order it to defend the aerodrome. As the Brigadier and his men attempt to fight their way past the monster, the Master places the final key in place and opens the transmat link to the Gaderene homeworld. Meanwhile, Whistler recovers from the trauma of his interrogation and escapes, killing McGarrigle as he goes.

Jo and Benton arrive at the aerodrome as the Doctor lands the Spitfire, and while Benton joins the rest of the UNIT forces, the Doctor confront the Master and tries to convince him to disable the transmat link. The Master refuses to do so, but as the link begins to stabilise Bliss announces that the Master's usefulness is over. She disposes of her human guise, revealing her true insect form, and turns on the Master, allowing the Doctor, Jo and Noah to escape. Whistler finds the Spitfire and takes to the air, but as he attempts to aid the Brigadier the Doctor uses Jo's R/T set to contact him and pass on new instructions. Whistler thus fires upon the transmat link, and manages to shoot some of the keys out of place, destabilising the link once again. The Master fights off Bliss and destroys her with his TCE, but as he enters the link to replace the damaged keys Whistler crashes his Spitfire into the energy beam. He ejects at the last moment, thus surviving, but when the Spitfire explodes the keys are destroyed and the energy feedback destroys the Gaderene invasion force on their homeworld. The death of Bliss results in the death of the telepathically linked embryos and of the monster as well, but while the villagers recover from their possession, Jo realizes the Doctor does not count this as a victory -- for the Doctor still remembers when the Master was one of his best friends, and the renegade Time Lord was still within the transmat link when it was destroyed.