On the eve of his 41st birthday, Harry Sullivan learns that, despite his moral objections to the work, he is to be transferred to NATO's chemical weapons development programme on Yarra. An inexplicable attempt is then made on his life by an East Indian strongman, and while recovering from the incident, Harry attends a Van Gogh exhibition and meets a young woman, Samantha, who seems to know more about him than she should. Samantha, who claims to do volunteer work for the Anti-Chemical Hazard Environment Society, invites him to visit her some time. While trying to sort out his feelings about his new posting, Harry accepts an invitation from his friend Teddy Bland, who is himself a senior researcher at Yarra, to spend a weekend at his home in the Hebrides. On his way back, however, Harry is run off the road by a Land Rover, and awakens in Castle Mackie, home of American neurologist Alexander Shire; there, he is shocked to overhear what appears to be his old friend Brigadier Lethbridge-Stewart calmly discussing classified details about UNIT operations. Harry then spots a mud-spattered Land Rover parked outside the castle and flees. Later, he meets Lethbridge-Stewart and tries to make an off-hand inquiry about Castle Mackie, but although he admits owning a cottage near the castle, Lethbridge-Stewart claims never to have visited the upstart Shire. Harry remains unaware that, while he was unconscious, Shire administered a dose of sodium pentothal to him and he revealed all he knew about his forthcoming work at Yarra...

Despite his reservations, Harry accepts his new posting at Yarra, but one day he accidentally overdoses a researcher with the antidote to the toxins she's been developing. Much to his surprise, this seems to cure her recurring migraines, and some time later she announces that, after months of trying with her husband, she is finally going to give birth. Harry begins to research these potentially beneficial side-effects of the antitoxin, but neglects his official work and attracts the wrath of the establishment's director, Conrad Gold. When three ampoules of the deadly nerve toxin Attila 305 disappear, Gold immediately orders the reluctant Teddy Bland to begin investigating Harry. Meanwhile, Samantha contacts Harry again and invites him to visit her at her home -- Castle Mackie. He is surprised to learn that Alexander Shire is her father, and, unaware that he is under investigation, accepts her invitation in order to learn more. Upon arriving at Castle Mackie, Harry stumbles across evidence that Shire apparently has access to surveillance devices planted in Yarra, and meets another of Shire's guests -- East Indian strongman Rudolf Rainbow, the man who tried to kill him on his birthday.

Samantha playfully pushes Harry into the castle pool, soaking his clothes, and Rainbow then locks Harry into the changing room. That night, still wet and shivering, Harry is taken from the castle and driven to a deserted croft by a stranger named Zbigniew Brodsky, who demands that Harry tell him everything he knows about the Attila antitoxin. When Harry refuses to do so, Brodsky flings one of the stolen ampoules into the croft and drives off, leaving Harry alone with the shattered ampoule in order to see its effects upon him. Harry wraps up the ampoule in his dampened blazer, hoping that the water will absorb the vapour, but he is already beginning to feel dizzy. Samantha unexpectedly arrives, looking for him, but she apparently falls victim to the toxin and loses consciousness; too weak to pull her to safety, Harry takes her car and drives to Lethbridge-Stewart's cottage to seek help. Lethbridge-Stewart is not there, however, and Harry finds map co-ordinates scribbled on a notepad by the phone. Lethbridge-Stewart's butler, Curly, turns out to be involved in the conspiracy, but Harry manages to evade him and flee, evading both Brodsky and his enforcer, Rudolf Rainbow. Brodsky, however, is convinced that Harry's survival means that he has vital information about the antitoxin, and is determined to extract it from him...

Harry gets back to his flat in London and sleeps, exhausted, for 19 hours. When he awakens, he prepares to report his experience to his superior, Rear-Admiral de Longpre, but before he can do so he is contacted by Shire, who claims that Samantha has fallen victim to the toxin and needs the antidote to survive. Uncertain what to do, Harry is about to explain all to de Longpre when he is arrested by Special Branch and charged with treason; the remains of the stolen ampoule have been found wrapped in his blazer. Sarah Jane Smith visits him in prison and reveals that she's recently interviewed Shire and Samantha for a piece she's writing on the anti-bioweapons brigade -- and that although this occurred after Harry's experience, Samantha was perfectly healthy. But now they have both disappeared, and so has Lethbridge-Stewart, which seems to implicate him in the conspiracy as well. Harry is determined to solve the mystery, but his solicitor advises him to pay more attention to his defense; not only has he been linked to the theft of the ampoules, but to the Van Gogh Appreciation Society, a front for the European Anarchist Revolution and its leader, known terrorist Zbigniew Brodsky.

While waiting to be remanded for trial, Harry tries to find the location of the map co-ordinates he recalls seeing in Lethbridge-Stewart's cottage, and finds them in the Summer Isles. He is removed from prison for interrogation by the British security forces, but their van is attacked and he is "rescued" by Brodsky and Shire. They believe that he now has no choice but to hand over the antidote to them, but while their guard is down he escapes and heads for the Summer Isles. There, on an ancient barrow known as the Bryg a Dyr, he spots Conrad Gold burying something beneath a rock, and once Gold has gone, Harry unearths the package to find that it contains microfiches of the classified work done at Yarra. Realizing that Gold is the real traitor, Harry contacts Bland and convinces him to arrange a stakeout at the Bryg a Dyr to catch the real criminals. He then leaves a note for the gang, ostensibly from Gold, telling them to meet Saturday at dawn for samples of the antidote, and contacts Gold, claiming to have photographs of Gold planting the stolen microfiches and threatening to blackmail him. At Saturday before dawn Bland and a Special Branch team arrive, and despite their suspicion of Harry he manages to convince them to wait for the conspirators to arrive. When they do arrive, however, when Harry sees the angry Brodsky slapping Samantha for some failure on her part, and is unable to stand by and allow this. His premature arrival bungles the operation, and most of the conspirators escape, but Gold is captured and Harry's name is thus cleared. Lethbridge-Stewart is eventually found, having been kidnapped by his butler Curly to keep him out of the way.

Harry makes a full report to the Admiralty and is advised to leave the rest of the operation in their hands, but he follows a hunch and attends the annual general meeting of the Van Gogh Appreciation Society at the Eiffel Tower. There, he finds Shire, Samantha and Brodsky, who inform him that he is no longer needed. Harry manages to escape from Rainbow by climbing out onto the struts of the tower, but he is pursued by Brodsky's other enforcer, Romanian acrobat Waldo Tedescu. Tedescu attempts to throw Harry from the Tower, but Harry overpowers him and escapes, and it is Tedescu who falls to his death. Harry returns to London, where Teddy Bland's sister Esther, waiting for him to return, promises she will take care of him.

Source: Cameron Dixon