A quiet, sunny summer's day in Perivale. A young man is washing his car, observed by a black cat, when something appears from thin air and chases him down the street. The young man vanishes in a flash of light, and the black cat runs off as the TARDIS materializes nearby.

Ace wanted to return and find out what her old friends were up to, but she'd forgotten how boring Perivale is. None of her friends seem to be around and experience has taught her just how pointless some of their "fun" used to be. She looks for her friends on Horsenden Hill while the Doctor notes with interest a set of hoofprints in the ground. Their next stop is the youth club, where the coffee bar has been boarded up and ex-Territorial Army Sergeant Patterson is teaching self-defense to a group of street youths. Young Stuart bests his opponent but Patterson taunts him until Stuart finishes him off, slamming his head into the mat as the Doctor and Ace watch in distaste. The class breaks up, and Ace inquires about her old friends, only to learn that most of them have disappeared recently. When Patterson recognizes Ace and starts chastising her for never calling home, Ace storms off in disgust. The Doctor seems more interested in the stray black cat which has been wandering around the exterior of the club -- he senses that someone has been looking at him through the cat's eyes...

While Ace looks for her friends in a local pub, the Doctor pops into the nearby grocer's, where owners Harvey and Len are wearily discussing the importance of staying open on Sundays in order to compete with the big-name markets. The Doctor points out the inherent barbarism of the "survival of the fittest" concept and leaves with an armful of cat food when the black cat pops out from behind a stack of tins, startling Harvey and Len. Shortly afterwards, Harvey finds that something has eaten his own cat. Elsewhere, Stuart is jogging to relieve his frustrations, when the man controlling the black cat selects him. Something chases Stuart down the street and he vanishes in a blaze of light.

Ace and the Doctor run into one of Ace's old friends, Ange, who's collecting for Hunt Saboteurs, and once again Ace is surprised to learn that many of her old friends have just disappeared recently. The Doctor, who's concerned with a different kind of hunt entirely, wanders off to a suburban side street and sets out a buffet of cat food; Ace, complaining about Patteron's attitude, realizes that the Doctor isn't listening to her and storms off to the nearby playground. There, she sees the black cat, and witnesses the materialization of a humanoid Cheetah riding a horse.

The Cheetah chases Ace through the playground, and the Doctor, trying to shoo away a curious dog from the cat food, hears her calling for help and rushes off to her rescue. He arrives too late; Ace, like her friends before her, has been transported to the planet of the Cheetah People, and is being hunted by the Cheetah which brought her here. The Cheetah runs her down near a copse where Stuart is hiding, and when Stuart frantically tries to shoo Ace away the Cheetah attacks and kills him. Deeper in the woods, Ace finds her friends Midge and Shreela and a young boy named Derek, who have been hiding in the woods for weeks.

Patterson tries to run but is brought down by the Cheetahs, who toy with him like cats playing with a mouse. The Master tells the Doctor that he needs help, but the Doctor ignores him and rescues Patterson, riding off on a horse with the terrified sergeant. The Cheetahs, deprived of their prey, grow restless, and the Master sends one of the black cats, "kitlings", back to Perivale to find fresh meat.

Ace urges the others to go on the attack, but Shreela is the only one willing to fight back with her. Their first attempt to capture a Cheetah fails and their second nets them the Doctor and Patterson. The Doctor explains that the kitlings have the power of teleportation, and seek out new worlds for the Cheetahs to hunt prey; the Master has been sending them to Earth and bringing people back to act as prey and keep the Cheetahs occupied. To make matters worse, the planet -- a very old planet -- seems to be breaking up.

The Doctor tries to lead his friends to a safe spot some distance away, but a milkman from Perivale is unexpectedly transported into their midst and panics, drawing the attention of a Cheetah pack. The Doctor urges his friends not to fight back, but, terrified, they do so and are split up. Ace throws a rock at the Cheetah pursuing her -- the same one which brought her to this world and killed Stuart -- and hits it in the head, seriously injuring it. Midge, separated from the others, is fought over by two Cheetahs and one leaves the other seriously injured; Midge takes the opportunity to kill it and takes its animal-tooth necklace as his prize.

The Doctor meets the Master in the ruins of an old building, and the Master explains that the people who built the ruins and bred the kitlings fell victim to the power of the planet and devolved into animals; now the same thing is happening to the Master, and he is relying on the Doctor to find an escape. The Doctor abandons him and finds Ace, who is tending the Cheetah which she injured earlier. The Cheetah speaks to her, identifying herself as Karra, and the Doctor worries that Ace is finding the Cheetah lifestyle dangerously attractive. He explains that the animals of this planet can use its power to travel to different worlds to seek prey, and can always return home; to get back to Earth, they need an animal whose home is Earth. He's unaware that the Master is nearby, listening to every word he says.

Ace runs off with Karra to hunt, ignoring the Doctor's pleas for her to return. The Doctor sets off in pursuit, while Ace and Karra seek fresh meat... but faced with the freshly killed remains of an animal, Ace begins to realize the raw danger of her new life. When the Doctor arrives and asks her to come home, Ace hesitates but eventually chooses him over Karra and the Cheetahs. She returns to the others and helps them to return home, to Perivale. As Derek and Shreela rush off to a reunion with their families, Patterson frantically denies that anything out of the ordinary happened to him and accuses the Doctor and Ace of standing by while he fainted and had a weird hallucination. He returns to the youth club for his next self-defense class, while the Doctor, realizing that things aren't yet over, asks Ace where Midge's home is.

Midge and the Master have arrived at Midge's family's flat in a council block, where the Master realizes that he is still succumbing to the raw power of the planet. He vows that if he is to become an animal he will bring the Doctor down as an animal. By the time the Doctor and Ace arrive, Midge and the Master have left, Midge's mother has been transported to the Cheetah world, and Midge's little sister's cat has been killed by the "bad cat the man brought".

Ace falls into a trance and realizes that the Master is on his way to the youth club. There, Midge confronts the self-defense youths and tells them that he's learned the secret of success, and he and the Master kill Patterson before the youths' eyes when the sergeant staggers into the club. The Doctor and Ace are too late to save him, but Ace follows the trail to Horsenden Hill, where Midge, the Master, and the self-defense class are waiting. The Doctor orders Ace not to fight -- if she gives in to the wildness she'll change forever. He leaps onto a waiting motorcycle and plays a game of "chicken" with Midge, which ends in a tie. Midge, battered and burned by the collision, gives up his own life in accordance with the Master's teachings -- the weak must die so the strong may survive.

The self-defense class closes in on Ace, but at the last moment Karra arrives and frightens them off. Karra then attempts to kill the Master, but he stabs her to death and heads off to collect the Doctor's TARDIS. There, the Doctor -- who survived the motorcycle collision -- confronts him, but the Master reveals that he's learned how to control the wildness of the old world and transports the Doctor to the Cheetah World to kill him once and for all. The Doctor nearly gives in to the wildness himself and tries to kill the Master, but he realizes what's happening to him just in time. As the last battle causes the Cheetah world to break apart completely, the Doctor returns "home", to the TARDIS.