When Lionel Cosgrove's mother gets bitten by a Sumatran Rat Monkey, she slowly turns into a zombie. Lionel takes the most reasonable course of action, and stuffs her in his basement. She escapes and zombifies more people. Lawnmower violence and ass-kicking priests ensue.
The scene in the park where Lionel takes the Zombie Baby for a walk was not in the original script. Shooting was completed early and with extra money left in the budget, so they used the extra time and money to improvise a new scene, which eventually became Jackson's favorite part of the movie.
When an archeologist sends a Sumatran Rat Monkey to New Zealand he had no idea the evil it would unleash.
When Lionel goes on a date with the beautiful Paquita he did not know that his obsessively controlling mother would stalk them. Not realizing where she was the Rat Monkey bit Lionel's mother infecting her. The next day she becomes grossly ill. Lionel calls for a nurse and Paquita ,for help, but it's too late and his mother dies in his arms. However Lionel's mother comes back to life and infects the nurse.
Lionel visits a doctor and asks for some tranquilizers so that he can subdue his mother and keep her controlled from eating others. During the funeral he meets Uncle Les who is trying to get control of Lionel's estate and the money that his mother had left him.
Lionel does his best to keep the living dead under wraps but after a trip to get his mother's body back from the grave things go from bad to worse. His mother infected a priest and several others and Lionel realized that just burying them was not enough and takes them all home.
Once home Lionel is blackmailed by his Uncle into giving him the house and all the money after Uncle Les discovers that Lionel's mother and several others are dead but still alive and Lionel had been trying to keep it a secret. Lionel is forced to take them all to the basement and chain them up.
Out of Lionel's control the infected nurse and priest have sex and while in the basement she gives birth to a demon baby. Lionel tries his best to raise the baby but is finally forced to lock it in the basement as well.
With the new house now under his control Uncle Les throws a huge house warming party inviting half the town. Paquita comes, hoping to see the now secluded Lionel but she has to fight off Les who tries to rape her. The zombies in the basement free themselves and start infecting all the party guests, turning them all into zombies.
That just happened
Lionel tries his best to escape with Paquita and at one point is forced to strap a lawnmower to his chest and mow down the zombies, in what maybe, the bloodiest scene in film history. With most of the zombies dead Lionel fights his mother who has now been turned into a super demon zombie. After his is sucked back into her womb Lionel uses the amulet, given to him by a mystic early in the film, to cut his way out of his demon mother.
With the demon mother and all the zombies dead Lionel and Paquita, the only survivors, walk into the night.