When Pete's (Oakes Fegley) grandmother dies, his newly widowed grandfather Ed (Robert De Niro) is struggling on his own. Pete's mother, Sally (Uma Thurman), and his father, Arthur (Rob Riggle), convince Ed to come and live with them. Unfortunately for the boy, his parents decide that Ed will take over Pete's room.

Upset by this news, Pete comes up with a series of pranks to get rid of his grandfather and get his room back. However, Grandpa isn't going down without a fight. He enlists his friends Danny (Cheech Marin) and Jerry (Christopher Walken) to help him come up with pranks of his own.

Based on the popular children's novel by Robert Kimmel Smith.