Tom and Liddy live at the home of her grandmother, who keeps a cat, a dog, and a canary. Tom comes home bringing a large package, and Grandma tells her dog to attack him. He gets past the dog to the kitchen and wonders where Grandma is. Liddy figures she's upstairs, locked in her bedroom and counting her secret stash of money. Tom shows her the new present that he received as payment for a plumbing job: a garbage disposal. He hooks it up beneath the sink and produces a bag of bones to feed into it to prove how good it is. The loud noise startles Grandma in her bedroom above.
That night, Liddy brings up food for Grandma and apologizes for the noise. Grandma thinks that Tom is trying to kill her so the couple can get her money. When Liddy suggests Grandma put her money somewhere safe like a bank, Grandma refuses to hear of it and tells Liddy she should have married someone responsible. Tom comes up and apologizes, and insists she'll love the garbage disposal after a while.
Later, Tom is secretly placing bets and lying to Liddy about a job interview he's supposed to be at. He notices that Grandma has received a letter from a pet home and slips it beneath her door. He then feeds more bones into the garbage disposal, and talks about how it's purring like Grandma's other pets. He starts reciting the line from the fairy tale about the giant and "Fee Fie Foe Fum." He talks loudly about grinding bones so Grandma, locked in her room, can hear him.
After Tom leaves for the day, Grandma goes downstairs to examine the garbage disposal. She finds a yellow feather and realize the canary's cage is empty. When Liddy comes in, Grandma tells her that Ted fed her pet canary into the garbage disposal. Liddy insists that the cage door is loose and the canary flew away, and it'll eventually return. Grandma doesn't believe it and assures her granddaughter that Tom plans to kill her off and grind the bones. When Tom comes home, Grandma runs back to her room and Tom denies killing the bird. When he tries to call an apology through the door, Grandma yells back and tells him that he'll never get her money.
Later, Liddy leaves for her job at the grocery store and Tom start chanting the fairy tale again. He goes to Grandma's door and tells her that he's going to take care of her cat and dog. As he walks away, Grandma peers at him through the peephole and sees that he's carrying an axe. He goes downstairs and she can hear him chopping at something on the carving block, and then running it into the garbage disposal.
Grandma goes downstairs later and her cat and dog are nowhere to be seen. She finds fur in the disposal and accuses Tom of murdering her pets. Even Liddy isn't sure, but Tom insists that Grandma set up the entire thing to turn Liddy against him.
After Liddy and Tom go to bed, Grandma sneaks downstairs and demands to know what the garbage disposal has eaten. She finds the axe beneath the sink, covered in dog hairs. However, then she goes out to the garage where the pets are right where she put them. She muzzled the dog to keep it from barking and alerting anyone to its presence. Grandma promises them that they can soon return to the house.
The next day, Grandma gives Tom and Liddy $500 and tells them to have a good time at the lake. She explains that she wants to be alone to mourn her departed pets, and to prepare to donate all of her money to the pet home. Once the couple leave on their trip, Grandma brings the pets back in and then pours some blood around the sink.
Grandma waits and, just as she expected, Tom comes back. He's snuck away from Liddy at the mall and has come back to get Grandma's money. He finds a lock of her hair on the sink, just where Grandma placed it, and the blood. Grandma comes in and says that she's set the whole thing up as a joke. When Tom comes at her, Grandma unleashes her dog. Tom trips on the blood and falls under the sink. He stares up in horror at the grinding blades of the garbage disposal while Grandma picks up an axe…
Later, Liddy comes home after realizing that Tom snuck away. When she looks for him, she finds Grandma sitting with her pets in the kitchen, saying she likes the garbage disposal after all.
The next day, the postman comes by and notices that Liddy and Tom are moving out. Tom is so shocked from his experience that he can't do anything but stare and mutter. As Liddy drives away with her husband and all their belongings, a happy Grandma proudly shows off her garbage disposal to the postman.