In 1957, explorer Stewart McAlden and his team smuggle a captured Sumatran rat-monkey, a hybrid creature that resulted from the rape of tree monkeys by plague-carrying rats, out of Skull Island. During the team’s escape from the island’s warrior natives, who demand the return of the creature, Stewart is bitten by the rat-monkey, resulting in his dismemberment and killing by his crew, who fear the effects of the bite. The captured rat-monkey is then shipped to Wellington Zoo in New Zealand.
In Wellington, Lionel Cosgrove lives in a Victorian mansion with his domineering mother Vera. When he was a child, Lionel’s father drowned trying to save him at the beach, and the incident has haunted him into adulthood. To Vera’s dismay, Lionel falls in love with a Spanish Romani shopkeeper’s daughter, Paquita María Sánchez, who is convinced the two are destined to be together. When the two visit the Wellington zoo together on a date, Vera follows them and is bitten by the rat-monkey which attacked one of the monkeys, and though she appears fine initially, the following day she grows more and more decrepit, culminating in her eating her own ear after it falls off in a custard during a lunch with colleagues, and later eating Paquita’s dog. She appears to die before reanimating as a ravenous zombie and killing the attending nurse Mrs. McTavish, who also returns as a zombie, before Lionel locks them both in the basement and keeps them sedated with animal tranquilizers. While visiting Paquita, Lionel is given a pendant for luck by her grandmother. Vera is able to break out of the basement and is apparently killed when struck by a tram.
At her funeral, Lionel tranquilizes Vera to keep her from attacking the mourners. Later, while returning to the graveyard to administer more of it, he is intercepted and beaten by a group of hoodlums who assume him to be a necrophiliac. Heavily drunk Void urinates on Vera’s gravestone. Vera bursts from her grave and attacks the hoodlums. In the ensuing commotion, Void and the local priest are killed and turned into zombies, forcing Lionel to hide them in the basement, too. After the nurse and priest copulate and produce a zombie baby, Lionel breaks up with Paquita to keep her safe. Shortly afterward, Lionel’s uncle Les arrives to wrangle with Lionel over Vera’s estate. Discovering the zombies, which he believes to be “stiffs”, in the basement, Les blackmails his nephew into giving up the house and his inheritance and invites his friends over for a housewarming party despite Lionel’s objections.
During the party, Paquita arrives to try to make amends with Lionel. She discovers the zombies in the basement, and Lionel explains to her all that has occurred. She is able to convince Lionel to administer poison to the zombies to finally kill them, but after injecting the zombies with it, he discovers the poison is animal stimulants, which revives them. They narrowly escape the now-enhanced zombies, who burst into the house upstairs and horribly and painfully slaughter the party guests. The guests subsequently reanimate and begin to attack the survivors, including Paquita and Lionel, who are separated in the ensuing mayhem, havoc and chaos. Lionel enters the house with a lawnmower and proceeds to purée the zombie horde within, while Paquita starts puréeing all body parts of any zombies she can lay her hand upon. Paquita then blasts the head of zombified Les, who has been killed by a now-monstrous Vera. Vera erupts from the basement and pursues them both to the rooftop as the house catches fire from a burst gas pipe.
As Vera corners them on the roof, Lionel confronts his mother and reveals that he witnessed Vera drowning his father and his lover in the bathtub as a child, and accuses her of lying to him all his life. Vera becomes enraged and swallows Lionel with an opening in her stomach before trying to kill Paquita. Lionel cuts his way out of his mother’s body with the pendant given to him earlier, causing Vera to fall back into the burning house. Lionel and Paquita escape the burning rooftop as the fire brigade arrives. They kiss and then walk away arm-in-arm.