When Lionel Cosgrove's mother Vera turns into a zombie, she kills a number of people, all of whom are also turned into zombies. Unsure of what to do about them, Lionel resorts to locking them in his basement. He leaves the house for a while to see his girlfriend Paquita, but while he is gone the zombies in the basement become restless. The zombified Nurse McTavish and Father McGruder seem to be attracted to one another, breaking free of their ropes and having sex. This results in McTavish becoming pregnant, though she gives birth only a short while after the intercourse. Lionel returns just in time to witness the birth of a grotesque mutant abomination which he names "Selwyn".
Lionel later takes Selwyn on a walk through the park, with Selwyn being locked inside his pram by barbed-wire mesh. This isn't enough to stop the freakish infant from escaping and rushing around the playground, almost attacking another child before Lionel catches him. Lionel lthen leaves Selwyn locked up with the other zombies in the cellar, though he and the other zombies escape during Les Kalkon's party after being injected with animal stimulant that was supposed to put them down. The ghouls go about slaughtering all of the party guests, with Selwyn brutally murdering Paquita's friend Rita.
Selwyn is the last character to die in the film, though his death isn't shown in any great detail. After Lionel slays his mutated mother, her giant carcass crashes through the house, causing an explosion as electrical lines and gas pipes are broken. Selwyn is last seen crying inside the kitchen as the house goes up in flames.