Willard Stiles is a meek social misfit who develops an affinity for rats. He lives in a large house with only his cranky and decrepit mother Henrietta for company. On his 27th birthday, he comes home to a surprise birthday party thrown by his mother, where all of the attendees are her friends. After leaving the party in embarrassment, he notices a rat in his backyard and tosses it pieces of his birthday cake.
A lonely boy named Danny Garrison befriends Ben, the leader of the colony of rats trained by Willard (1971 film). Ben becomes the boy's best friend, protecting him from bullying and keeping his spirits up in the face of a heart condition.