Marpling; a quiet English village, in the year 1933. The young Liam Jarrow idolises his late father, who died fighting in the Great War, and he intends to join the army himself as soon as he’s old enough. His friend, local dentist Charles Rigby, tells Liam war stories from his own experience to discourage him, but soon realises that they’re having the opposite effect. To change the subject, he shows Liam something odd which he’s been keeping in a box in his shed since he found it in his garden the other day; it appears to be a small black metal tube, but it’s oddly warm and alive to the touch. Liam inadvertently disturbs a hidden wasps’ nest while studying the strange object, and as he and Rigby flee Liam drops the tube, breaking it. Rigby locks up the shed, intending to wait for the wasps to calm down -- but when he returns the next morning, the wasps press up against the shed window, smash through the glass and pursue Rigby back to his house. As they crawl over his face, he screams for help... Some time later, Liam’s American stepfather, engineer Tom Carlton, arrives at Rigby’s for a dental appointment, but as he settles into the chair he notices that Rigby appears ill -- and then Rigby turns on him and spits a stream of wasps out of his mouth...

The TARDIS materialises on the village green, and moments after emerging, the Doctor, Anji and Fitz are confronted by the scandalised spinster Miss Havers, who assumes that they’re gypsies and orders them to clear off out of her village. The local black sheep Hilary Pink happens by and deliberately angers Miss Havers by inviting the strangers to his home, a manor house he shares with his elder brother “Squire” George Pink. The Squire is more rigidly upper-class in attitude than his wolfish brother, and politely welcomes in Hilary’s new friends -- but keeps a wary eye on them nonetheless. Fitz catches a glimpse of someone hiding in the garden, and the Doctor casually mentions that they were followed from the village. He doesn’t seem concerned; whoever it is will show themselves when they feel the time is right. The man in the garden, Jode, returns to the woods to report the blue box’s materialisation to his associates Kala and Fatboy. Their mission is already difficult, as the device they’re looking for was designed to avoid detection, and now it appears that there are illegal time-jumpers in the area. The blue box is awash with temporal energy, and this is making it even harder for Fatboy to locate the object they were sent to look for...

While Hilary and his friends share a picnic on the manor lawn, the Squire heads into town to speak to Tom Carlton about the reconstruction of St Cuthbert’s church roof; but when he arrives at the crossroads, he finds Carlton lying dead in the ditch. The Squire calls on the Doctor for help, and the Doctor studies the body and determines that Carlton died of anaphylactic shock after being attacked by a swarm of wasps. But although he must have died in agony, there are no crushed wasps in the vicinity; he must have been killed somewhere else, and the body moved. While the Squire stays with the body, waiting for the authorities, the Doctor and his friends accompany Hilary to tell Gwen Carlton and Liam about Tom’s death. Liam loathes Hilary, who was a conscientious objector during the war; but there is history between Hilary and Gwen, and he feels he should be the one to break the news. The Doctor notes Liam’s odd reaction to the news that his stepfather was killed by wasps after going to Rigby’s, and when he questions the boy and learns about the wasp nest in Rigby’s shed, he decides to investigate.

Miss Havers tries to put the unpleasant incident on the village green out of her mind by doing volunteer work at St Cuthbert’s, and she’s almost succeeded in doing so as she returns home. But on the way back she sees Rigby staggering across the fields in great pain, and when she gets close to him, she sees that he’s holding a black tube in his hands... and is surrounded by wasps. Reverend Fordyke is sorting out the hymnbooks in the church when Miss Havers runs back in, her hand clamped to her mouth as if she’s going to be ill; he then hears screaming from outside, and rushes out to see a swarm of wasps sting the verger Williams to death. Rigby is standing by the manse, watching... and changing.

The Doctor takes Fitz and Liam to Rigby’s to find out if there is any connection between the wasp incidents, and finds that the shed window has been broken from inside -- and the lock has been torn off from the outside. Inside, the Doctor finds a shard of the broken device lying inside an abandoned wasps’ nest, and pockets the fragment for later study. He then breaks into Rigby’s house to learn more, but Rigby has returned, and he holds a gun on the Doctor and his friends, speaking through gritted teeth as he orders them out of his home. The Doctor sends Fitz and Liam to safety and tries to speak to Rigby, but realises that he will get nowhere and is forced to leave before Rigby gives in to the compulsion to shoot him. Once outside, he is attacked by Jode and taken to the woods for questioning; Fatboy had detected a subsidiary time-trace moving in on the primary one, and Jode and Kala have concluded that the illegal time-travellers are here to steal the device. The Doctor deduces from their clothing and technology that they are from Earth’s future, and are searching from something; however, they won’t tell him what the device actually is. He distracts them and escapes, having learned more from them than they from him; despite herself, Kala is impressed.

Miss Havers seems fully recovered from her shock, and Fordyke thus reports Williams’ death to the Squire. The Doctor then returns, tells his story and reveals that he slipped the fragment from the shed into Fitz’s pocket to keep it safe; Fitz is somewhat hurt when he realises that this is probably why the Doctor was so insistent he get to safety. Upon closer examination, the fragment seems indefinably organic; not just damaged, but hurt, and in pain. The Doctor decides to break into Rigby’s house under cover of darkness to look for the rest of the device; Fitz reluctantly agrees to accompany him, but Anji wants no part of such a dangerous mission, and Hilary has been drinking ever since returning from Gwen’s. As the Doctor and Fitz leave, Anji starts to become concerned about Hilary’s drinking, and the Squire admits that Hilary and Gwen were more than just friends while John Jarrow was off at war. Gwen was devastated when John died, and Hilary left town to wander the world -- and by the time he learned of Liam’s birth, Gwen had already remarried. Ironically, Liam despises the cowardly Hilary and idolises the brave man he believes was his father. As the Squire prepares for bed, he inquires after the Doctor and Fitz -- and Anji, caught off guard by his honesty, admits the truth. The Squire hears only that they’ve gone to break into Rigby’s home and take something; he’d feared that Hilary’s friends would be untrustworthy, and is appalled to realise that he’s allowed thieves into his home. Anji tries to explain, but the Squire refuses to listen and calls the police to report a burglary in progress. Hilary, determined to show up his brother and the authorities, takes Anji to Rigby’s to warn their friends.

The Doctor and Fitz find Rigby standing immobile in his living room as wasps fly in and out of his open mouth. The fragment in the Doctor’s hand draws him to the rest of the device, which Rigby has stored in a metal box, unknowingly shielding it from the temporal squad’s scans; but when the Doctor opens the box, Rigby snaps to life and attacks him. Anji and Hilary arrive as they struggle, and when Hilary intervenes, Rigby coughs up a swarm of wasps over Hilary’s face. Rigby flees with the device, and as Hilary writhes in agony, the Doctor tells Anji and Fitz that they have to kill him -- but Hilary dies of his own accord, and with his last words he begs Anji not to tell Liam the truth. The temporal squad arrives, too late, and the frustrated Kala orders the Doctor to leave the area, as he’s a rogue element complicating her operation. But he sees it the other way around. Before he can question her further about her mission, the police arrive; Anji insists upon remaining with Hilary’s body, and thus the police find and arrest her, the Doctor, and Fitz, as the temporal squad flee. Despite Jode’s urgings, Kala refuses to declare the mission a failure yet, but if things get any worse she’ll have no choice but to resort to Plan B -- and sterilise the entire area with a nuclear bomb.

Inspector Gleave isn’t happy about being woken in the middle of the night, to be presented with a bizarre murder case and a suspect who claims that the dead man was killed by superintelligent wasps affected by an artefact from the far future. But he’s taken aback when the Doctor seems more frustrated by the delay of arrest than worried about potential murder charges. The Squire blames himself for Hilary’s death and posts bail for the others, but the Doctor refuses to leave without attending Hilary’s autopsy, and his persistence wears Gleave down. Anji and Fitz return to the manor, where Miss Havers stops by to offer the Squire her condolences; as usual, she’s received the bad news almost immediately. The Squire is trying not to show his grief... but when he next sees Rigby, there will be trouble.

Miss Havers continues on to the church, where Rigby is sitting quietly in a pew; Fordyke tried to speak to him earlier, but retreated in fear when he saw a wasp climb up Rigby’s shirt cuff. Miss Havers approaches Rigby herself, but for some reason he suddenly runs away when she speaks to him. He knows that he’s being pursued, and he latches onto the fading memory of a friend -- a boy named Liam. Seeking help, he breaks into Liam’s house, knocks Gwen aside with a blow and advances on the terrified Liam. Jode, Kala and Fatboy trace the signal from the device to Liam’s home, but when they enter and attack Rigby, their stun-weapons only seem to anger him. In the ensuing struggle, Fatboy stumbles into the kitchen stove, starting a fire. Kala drags Gwen to safety before she and her companions flee, but the house goes up in flames -- and when Anji and Fitz see the smoke and rush to help, they find Miss Havers and the other neighbours standing around, unable to help as the house burns to the ground with Liam apparently trapped inside. Meanwhile, Jode demands that Kala declare the mission a failure, pointing out that half of the synthetic skin on Fatboy’s face melted away in the fire. She offers a compromise, giving Fatboy the priming code word, but ordering Jode not to activate his timer just yet. First she wants to speak to the Doctor.

Professor Jacobs has already conducted an autopsy on Williams, and has found dead wasps in his mouth and nasal cavities. To Jacobs’ surprise, the Doctor takes over Hilary Pink’s autopsy, and when he cuts open Hilary’s abdomen he finds that the intestines have mutated into something unhuman. Living wasps then crawl out of Hilary’s intestines, and the body begins to shake as the wasps rewire its central nervous system. The Doctor captures a wasp and sets fire to the body, killing the rest of the swarm. He informs the shaken Jacobs and Gleave that the wasps were trying to rebuild Hilary’s cells, transforming him into an entirely different life form -- but as with Carlton and the verger, the venom of their stings killed Hilary before their work was complete. Gleave gives the Doctor a ride back to town and drops him off by the TARDIS to analyse the captive wasp, while he continues on to the Squire’s manor. There, Anji and Fitz are arguing about the Doctor’s behaviour; Fitz believes he wouldn’t have urged them to kill Hilary lightly, but Anji isn’t so sure. Does the Doctor really care about the people involved in this incident, or is it all just a game to him? Miss Havers then arrives with mixed news; the Carltons’ home has burned to the ground, but there were no bodies in the ruins. Liam must still be alive... but he must be with Rigby.

As the Doctor approaches the TARDIS, Kala contacts him and admits the truth; Fatboy is really an android with a nuclear bomb in his chest, and she has orders to detonate him if the mission fails. The Doctor allows her into the TARDIS, which is more technologically advanced than she had expected; stunned, she finally admits that the device is an experimental weapon from her own time, stolen by illegal time-jumpers who were subsequently vapourised by a fault in their equipment. She doesn’t know what it actually does; just that it could destroy the planet if activated improperly. The Doctor warns her that detonating an H-bomb in the middle of England in 1933 could change the course of history, but she believes that it’ll all be the same in 3000 years. The Doctor, however, is sure that Time doesn’t work that way. Almost sure. In any case, it certainly won’t all be the same for the people of Marpling, and he realises that Kala is having doubts and came to him for a second opinion. When he analyses the wasp from Hilary’s body, he finds that it is biologically no different from any other wasp -- but when he studies the fragment from the shed, he realises that the device is a storage medium for a bio-psionic energy field. The weapon is meant to be dropped behind enemy lines, where the psionic force will transform the enemy from within; but due to the damage it has suffered, it has transferred itself into the swarm, and the wasps are following its programmed imperatives to genetically re-engineer the humans they encounter. Most of their victims have died of shock and cardiac arest before the change is complete, but the wasps are learning with each victim, and if they succeed in spreading the psionic infection, it could destroy all life on Earth.

The Doctor analyses the weapon’s psi-wave, develops a counter-frequency and encodes it into a molecular chain which he bonds to the carbon dioxide in the TARDIS fire extinguisher. He and Kala emerge from the TARDIS to find Miss Havers waiting outside; as Rigby is on the run from the law, she is willing to put aside her personal distaste for the Doctor to assist him. Fitz, Anji and Gleave then arrive to report that Fordyke has been attacked by wasps in the church -- and while fleeing, he saw Rigby driving towards the railway station in Hilary Pink’s Bentley, with the terrified Liam in the passenger seat. If Rigby gets to the station first, he could infect everybody on the train. Kala decides that the situation has gone critical; she will have to give Jode the order to activate Fatboy. The Doctor knocks her unconscious, telling the appalled Fitz about the threat of the bomb; they’ll have to take Kala to the station to prevent her from getting back to Jode. Meanwhile, Jode waits impatiently for Kala to return, ignoring Fatboy’s comments that the Doctor’s police box appears more advanced than even their own time-travel equipment. It appears as though Kala has run out on them; Jode may have to give up on her and activate Fatboy himself...

Gleave and the recovering Kala accompany the Doctor and his companions to the railway station, where they catch a glimpse of Rigby boarding the train with Liam just as it starts to pull out. The Doctor, Anji and Kala get aboard with the Doctor’s psi-spray, to find Rigby has mutated further; he is becoming more insectile with every moment. Rigby clambers onto the train roof, still dragging Liam along with him; the Doctor follows him, but Kala pulls the emergency cord and Rigby overpowers the Doctor while he’s caught off guard by the sudden deceleration. Rigby flees into the next carriage, but the Doctor pursues him and threatens him with the psi-spray, trying to get him to listen to reason. He knows that Rigby won’t kill Liam; he hasn’t so far, because a part of him is still Charles Rigby, and he remembers that Liam is his friend. Rigby retreats, taking Liam with him, and sends wasps to surround the carriage -- but the Doctor lets them in and uses the psi-spray against them. It works; the gas nullifies the weapon’s effect, and the wasps become an ordinary, albeit dazed, swarm.

The Doctor, Anji and Kala emerge from the train along with the panicking passengers, but Anji loses her temper when the Doctor shows more apparent concern for the wasps they’re treading on than he did for Hilary or Liam. She storms off to be alone for a moment -- but is captured by Rigby, who intends to use her as a hostage should the Doctor try to follow him. By the time the Doctor realises that she’s gone, Kala has lost her temper as well; she will waste no more time on the Doctor, and intends to order Jode to activate Fatboy. The Doctor finally realises what she’s saying -- Jode has the power to activate the bomb regardless of whether Kala gives the order or not. The Doctor will have to trust Anji to take care of herself, while he returns to Marpling with Kala, to find Jode and stop him. However, he’s too late. Jode has given up on Kala, and he and Fatboy have traced a secondary psionic source to St Cuthbert’s. There, Jode gives the code to activate the timer, and sets off to return to his own time while Fatboy, now metaphorically ticking, hides in the scaffolding around the ceiling.

Fitz and Gleave return to Marpling to search for the nuclear bomb. The Squire and Gwen Carlton also arrive, seeking something to do to avoid being alone in their grief. Miss Havers claims that the church is empty, but Fitz catches a glimpse of Jode running away out back, and he and the Squire give chase. However, Jode runs into Rigby first -- and Rigby is now almost entirely insectoid, and has grown extra segmented limbs to hold onto Liam and Anji. When Jode tries to shoot the monster with his stunner, this enrages it, and it tears him apart. The Doctor and Kala arrive to find Fitz and the Squire standing over Jode, who dies without telling them where Fatboy has hidden. According to Jode’s chronometer, the bomb will go off in 34 minutes; according to Kala, Fatboy is a last-resort device and he cannot be switched off once activated. His intelligence unit will be overridden by the bomb’s programmed directives, and he will fight off any attempts to defuse him; however, in the last stage of the countdown, all of his motive power will be diverted to prime the detonator. But this will give the Doctor only four minutes in which to defuse him -- and there’s still Rigby to deal with. Soon his transformation will be complete, and the psi-wave will spread across the Earth like an infection, destroying all life on the planet.

The wasps suddenly swarm away from Jode’s body, heading for the church; Rigby has recalled them while he undergoes the final stage of his transformation. Fitz then remembers that Fordyke was attacked by wasps in the church, which is where Fitz saw Jode running away; the Doctor realises that Rigby must have hidden in the scaffolding, and that Fatboy has planted himself there to destroy the infection at its source. Kala picks up something of the Doctor’s enthusiasm, and although she now has the opportunity to return to her own time, she follows him to the church instead. There, he orders the others to evacuate -- but instead, Miss Havers spits wasps at him and Kala. She hasn’t been following events just because she’s the local gossip; she was the wasps’ only other successful conversion, and ever since she met Rigby in the fields outside the church she has been his eyes and ears amongst the investigators. The Doctor empties his canister of psi-spray over her and Kala, killing Miss Havers and saving Kala’s life, but this means that he will have to face the monster that was Rigby without any weapons.

As Rigby enters the final stage of transformation, Anji tries to urge Liam to flee, but he is paralysed by terror -- and shame at his own fear. When he finally tries to move, the monster lashes out at him, and in his horror Liam shouts out that he’s not Mr Rigby; Rigby was a good man, who knew his father. Indeed he did; and the monster reveals that Liam’s real father was the coward Hilary Pink. Bearing Hilary’s last words in mind, Anji tells Liam that this is a lie, and the enraged Liam attacks the monster, who knocks him off the scaffolding. The Doctor arrives just in time to save Liam, and sends him back down to his mother while he confronts the monster. He calls on Fatboy to show himself, and informs the monster that in less than nine minutes, the church and the surrounding countryside will be levelled. He also shows it the fragment he took from Rigby’s shed; the device is still the main housing for the bio-psionic force, and as long as it remains damaged, the monster will never feel complete. Perhaps there is still a shred of Charles Rigby left; in any case, the monster finally hands over the rest of the device, trusting the Doctor to repair it. But instead, the Doctor drops the device from the scaffolding, and it smashes to pieces on the nave floor. The bio-psionic force dissipates, the wasps revert to normal, and the monster falls from the scaffolding to its death. Fatboy must still obey his programmed imperative and detonate, even though the danger is past, and when the Doctor tries to make him see reason, Fatboy interprets his approach as an attack and defends himself. Before he can kill the Doctor, however, he enters the final stage of the countdown and stops moving. The Doctor recovers, and uses his sonic screwdriver to open up Fatboy’s chest -- and even though a wasp lands on his hand and stings him at a critical moment, he still manages to defuse the bomb.