Tony's father Sam, abducted by aliens three years earlier, returns to earth and seeks out his wife and son, but Rachel has since been living with Joe and the reunion is awkward. Joe doesn't trust Sam, and Rachel can't quite decide what her feelings are for her two men. Sam is not the same as when he left, and he begins affecting Tony in frightening ways.
A man who was abducted by aliens returns to his family three years later, but his presence affects them negatively.
- Sam Phillips (Philip Sayer) and his child Tony (Simon Nash) are playing outside their farm. The father is abducted by a strong light.
Three years later, the light returns, and plants a seed. A half-human, half-alien creature (Tim Dry) grows up, and when it moves it is run over by a car. The driver Ben (Robert Pereno) is attacked and killed when he looks for the crash victim. Jane (Katherine Best), his companion, is also killed by the hybrid creature. The monster moves to a cottage nearby and attacks and impregnates a woman (Susie Silvey) living there; when she returns to consciousness, the alien monster decomposes into dust, and the woman's belly rapidly and painfully grows to a gargantuan size and she gives birth, vaginally, to a fully formed and bloody Sam, who is connected to her by an umbilical cord like a baby is to its mother, before dying. Sam washes the blood off, steals Ben's clothes and drives his car without bothering to get rid of Jane's corpse, which will be found by a lorry-driver (David Cardy).
Sam seeks Tony, who lives in an apartment building in London, with his mother Rachael (Bernice Stegers), her new boyfriend Joe Daniels (Danny Brainin), and a French au-pair Analise Mercier (Mariam d'Abo). Rachel and Joe are professional photographers and share a studio in town. For the past few night, Tony has nightmares where he wakes up soaked in blood, but it's not his, as the family doctor discovers. Sam picks Tony up from school, until Rachel finds them. Although Joe doesn't like it, as he intends to marry Rachel, Sam goes to live with them, saying he can't remember anything from the day he vanished. Tony sees him eating his pet snake's eggs and runs from him. Sam goes after him, talking to him smoothly, and drinks his blood. Sam tells Tony that he has been to another planet and plots to spread his seed for his new adopted alien race. He drinks more of Tony's blood and infects him with millions of microbes.
Rachel finds Jane's photo in Sam's clothes, but he can't remember her either (so he claims). Tony discovers he has certain powers now, so he sends a human-sized toy soldier to kill their nasty neighbour Mrs Goodman, in revenge for killing his pet snake, and a teddy-clown becomes a human-like clown.
Sam and Rachel both decide to visit their former residence, the farm, while leaving Tony in Analise's care. However, she brings Michael, her boyfriend, and they make love. Tony demands to play hide-and-seek with her. She does so, only to be knocked out by the clown and used as a womb for the alien eggs; Tony sends a toy tank to kill Michael. He discovers a cocooned Analise and runs away, but a black puma kills him. The building keeper, Mr. Knight, is also killed when Rachel asks him to watch Tony, as nobody answers the phone at home.
Sam and Rachel make love at the abandoned farm, but she gets afraid because his skin starts to bleed and decompose. Joe has taken Tony there. Sam and Tony go up a hill towards the alien light. Sam has now taken the form of an alien, and his scream sonic pulse kills Joe. Along with Tony, Sam enters the light and returns to the alien world.
Rachel sits down in the field where Tony and Sam left, and the next day returns to her apartment, only to be seen full of eggs. She picks up an egg, only to be killed by the same creature that impregnated the woman in the cottage as her apartment door slams shut behind her.