Mischievous Dennis Mitchell (Mason Gamble) makes the life of neighbor George Wilson (Walter Matthau) miserable with his overactive energy and inadvertent troublemaking. Because his parents must leave town for work and can't find a baby sitter for Dennis, they ask George and his wife, Martha (Joan Plowright), to take care of him. But when burglar Switchblade Sam (Christopher Lloyd) breaks in to steal George's gold coin collection, he takes Dennis as a hostage, and crusty George must save the boy.