The Doctor develops an interest in a new street drug called warlock, which may be related to a chemical formula smuggled out of the Soviet Union shortly after World War II. However, none of his tests produce results; it’s almost as though the drug doesn’t want to be analysed. He therefore sends Benny to New York City, to infiltrate the International Drug Enforcement Agency and obtain either a sample of the drug or the results of their own studies. After being accepted into IDEA, Benny is sent undercover to a drug deal where warlock is being traded, but she is unaware that an undercover cop, Creed McIlveen, is also present at the deal. The Mayan dealers suspect that one of their buyers is a narc, and slip warlock to everyone present, claiming that they will use the spirit of the drug to sniff their enemy out. The drug seems to amplify people’s perception and body language, and a cold breeze seems to circle the room, almost like a psychic lie detector. In the end it settles on Russell, the Mayan brothers’ gofer, who is doomed by his own lack of self-esteem and his terror that he will be erroneously singled out. The Mayans then use the power of warlock to deny his existence, killing Russell by amplified social ostracism. Before the deal goes down, however, one of the Mayans spots IDEA officers Raymond and Christine Bowman moving into position, and pulls a gun. The real drug dealers are all killed in the exchange of gunfire, and much to Creed’s disgust, IDEA takes the samples of warlock for analysis. Creed is given leave to recover from his experience.

Back on Allen Road, Ace is contacted by two counter-culture rebels, Shell and Jack, who wish to warn her and the Doctor about the presence of an animal experimentation lab in the countryside nearby. She joins Shell and Jack for drinks at the local pub, but isn’t convinced by their horror stories about the lab farm, even when they point out the blond man who supposedly kidnaps household pets for the laboratory. When she leaves the pub, however, she finds that her cat Chick and Jack’s dog Sheba are missing from their cars. She thus has no choice but to go to the lab to rescue her cat, but once there they find the blond man Sean waiting for them. Jack explains that he struck a deal with Sean in order to lure Ace to the lab to see what really goes on here -- but Sean has in fact lured them into a trap, as he and his team wish to experiment upon humans and have lured people here who won’t be missed if they happen to vanish. Dieter, the project leader, threatens to charge them all with criminal trespassing -- unless they volunteer, of their own free will, to participate in the warlock drug tests as human guinea pigs.

Benny is put to work analysing warlock at the IDEA headquarters in the King Building in New York. She sends her findings, or lack thereof, to the Doctor, unaware that her transmissions are being monitored by Hank Harrigan Junior, the head of IDEA; he is aware that she is an impostor, and is playing her along in order to find out who her real boss is. Since she will soon have to be replaced, Harrigan decides to enlist Creed. The Bowmans blame Creed for opening fire during the drug bust, trying to deny their own responsibility for the fiasco; when Harrigan orders them to bring Creed in they decide to make it clear to Creed that they are the ones in charge. Creed, however, has spent his leave thinking about his lover and his brother, both police officers who were killed in the line of duty, and trying to come down from the drugs he took while undercover. Just before the Bowmans arrive he takes a pill of warlock which he had kept after the bust. The Bowmans thus fail spectacularly to intimidate him, due to his pinpoint interpretation of their body language; he only agrees to accompany them to IDEA because he feels like it. Harrigan, infuriated, tells Raymond the truth about his wife -- although Christine had claimed that she’d killed her father as a child because he had abused her, in fact she’s a psychopath who just did it because she felt like it.

The machine which Benny had been using to analyse the warlock samples prints out a warning, supposedly from the Doctor, and she therefore gives her “colleagues” the slip and returns to England -- where the Doctor claims that he didn’t send any warning. Since IDEA has proven to be a dead end, the Doctor decides to contact Vincent and Justine Wheaton, hoping that Justine will be able to locate her old supplier Mrs Woodcott, who knows more about warlock than any other living person. But Vincent has just had a dream that the Doctor returned and caused Justine to leave him, and when the Doctor really does show up, Vincent flees in terror, taking Justine with him. Justine convinces Vincent that they owe everything they have to the Doctor, and when he eventually agrees to speak with the Doctor and find out what he wants, he finds that his old empathic powers are starting to return to him.

Unfortunately, IDEA tapped into the Doctor’s computer system while he communicated with Benny, and they have learned of the link between Justine and Mrs Woodcott. Creed and the Bowmans are sent to England to capture Justine, but soon after they arrive, their colleague Webster finally manages to hack into the classified data files of the King Building’s former occupant, the Butler Institute -- and learns the truth about Vincent Wheaton. Harrigan sends him to England, but he catches up with the rest of the team too late. Vincent and Justine are on their way to Allen Road, but they stop for lunch in Canterbury first, and the team catches up to them there. Webster shoots Vincent with a tranquilliser dart, but as he passes out Vincent touches Raymond and taps into the fierce hatred Raymond feels for his wife. This hatred takes on physical form as a fireball which incinerates Raymond, Christine, the news team sent to film the capture, several nearby tourists, and Canterbury Cathedral. Creed and Webster flee with Justine and the unconscious Vincent. Harrigan himself comes to England to supervise the rest of the operation, and tells Creed to take care of himself; Harrigan knows he is dying, and he wants Creed to take over IDEA when Harrigan has passed on.

Ace, Shell and Jack are given overdoses of warlock and strapped into chairs in the “barn”, while a video camera records their responses. Shell begins to panic, convinced that the sacred spirit of warlock has been desanctified by the experiment, and that terrible things will happen as a result. As Ace looks about herself, desperate to escape, her gaze falls upon the caged animals awaiting experimentation… Some time later, Tommy Hunnicutt arrives in the barn to carry out the first of the experiments, although in truth they’re just an excuse for him to take out the humiliation and pain of his life by torturing the animals. But as Ace, Shell and Jack loll in comas nearby, the black cat Tommy has chosen for the first experiment claws his hand and runs into the nearby alcove -- and when Tommy follows it, it sets a trap for him, clawing open the water cooler bottles and severing the lamp’s electrical cord. In the darkness, Tommy doesn’t realise the cord has been cut, and when he tries to switch on the lamp he electrocutes himself. The black cat then releases the other animals, but they are all soon recaptured. Tommy’s sister Pam, who also works at the lab, leaps to the correct, incredible conclusion in her grief, and begins to conduct particularly brutal “experiments” on the animals, knowing that soon she will find the one containing the mind of the human who murdered her brother…

Vincent is held captive in a life support tank to prevent him from using his powers again, while Creed takes Justine to London to search for Mrs Woodcott. He tries several times to make a pass at Justine, but she ditches him as soon as she can, and sets off to warn Mrs Woodcott that she’s being hunted by IDEA and enlist her help to rescue Vincent. Instead, she is captured by street thugs and taken to one of Paulie Keaton’s brothels, where she is to be put to work after her pregnancy is terminated. Creed, unable to locate her by ordinary means, takes warlock and heads out into the streets, where the spirit of warlock guides him to Mrs Woodcott and to a Japanese girl wearing Justine’s jacket. Creed and Mrs Woodcott force the girl to take them to the brothel, where they rescue Justine, and Creed then goes to Keaton’s favourite restaurant and uses body language to intimidate the gangster into flinching away from him. Keaton’s weakness is exposed before his fellow gangsters, and Creed leaves him, knowing his days are numbered. Creed has saved Justine’s life and her baby, and, grateful beyond words, she accompanies him to Buckingham Palace, where the guard waves them in without looking, assuming from Creed’s body language that this is the King and his latest conquest. Creed and Justine make love in the royal bedrooms.

The power of warlock has transferred Shell’s mind into the body of an aristocratic white cat, and it enables her to communicate telepathically with Mrs Woodcott and tell her where the animal farm is located. Unfortunately, before help can arrive, Pam kills both Chick and Shell, and the dog containing Jack’s mind, mad with grief, breaks out of his cage, tears out Pam’s throat and flees into the fields outside the farm. Shell’s human body comes back to life briefly and throttles Sean to death before collapsing, dead. The Doctor and his friends arrive at the farm too late to save Shell, but by giving warlock to Ace and the black cat they transfer Ace’s mind back into her body. Dieter claims that he just found the comatose bodies of Ace and her friends in the grounds and helped them inside; the Doctor and Ace are unable to prove their claims against him, but Mrs Woodcott is certain that warlock will exact its own revenge against him in due course.

The Doctor and his friends rescue Vincent from Webster, who is happy enough to let him go, as he was never at ease with kidnapping a civilian. Back at Allen Road, as the Doctor puts Jack’s body into the life support tank, Benny tries to convince Creed not to get between Justine and Vincent. Before the situation can be resolved, Harrigan arrives and holds them all at gunpoint, having shot Webster. He now reveals that, as the Doctor had suspected, he is not Hank Harrigan Junior, but Senior. The Soviet scientist Ludmilla Serebrennikov supplied samples of warlock to him after World War Two in the hope of striking an emigration deal, but while she ended up in Stalin’s gulags, Harrigan experimented with the drug and found that it was able to transfer his mind from his own elderly, dying body into the body of a young, healthy volunteer GI. As Harrigan Junior, he founded IDEA in order to keep the war on drugs in the public spotlight, guaranteeing himself a regular source of income while keeping an eye on the progress of warlock. Now, having arranged for Creed to take his place once he dies, he plans to use warlock to transfer his mind into Creed’s body.

But before Harrigan can act, Vincent touches him and amplifies the power of the warlock within him -- and, as the Doctor was starting to realise, warlock is revealed to be an alien intelligence which crashed on Earth long ago, and which has been roaming the world in drug form searching for an escape. With the help of Vincent’s powers, the warlock immolates Harrigan’s body, providing it with the final boost of energy it requires to escape from Earth at last. But when Justine’s husband burns a man to death at his mere touch, she instinctively rushes into Creed’s arms, and that act brings her marriage to an end. Angry and bitter, Vincent departs from Allen Road without saying goodbye, and drives off into the distance -- accompanied only by a dog he found roaming the fields outside, and which he decides, for some reason, to call Jack.