Jimmy's mother moves out of her house and into an apartment, and calls to say she can no longer keep his childhood dog, Happy. Jimmy wants to take the dog, but can't because Greg's allergic. After Kim tries to convince Greg to take it in anyway, Greg tells the truth -- he's not allergic to dogs, he just doesn't like them. Kim ignores Greg's wishes, and lets Jimmy and Christine keep Happy there. To get even with her, Greg gets something Kim doesn't want around: a go-cart. Happy arrives, but is hardly the dog Jimmy remembers; he's more like Cujo. Kim admits to Greg that she made a mistake, but still doesn't want to tell Jimmy to get rid of it -- even though Christine agrees it needs to go. Happy gets loose and chases after Greg while riding his go-cart, causing him to have an accident. But it's not Greg who gets hurt. While at the vet's, Greg, Kim and Christine are finally able to convince Jimmy that Happy is too aggressive to keep. The vet tells Jimmy that Happy is a goner and should be put to sleep, but then, miraculously, he pulls through. Only now Happy is twice as aggressive as before... and incontinent. Jimmy discovers that his mother isn't moving and lied to get rid of the dog. Greg manages to calm down Happy, and shares the story of how he came to hate dogs. Jimmy finally finds a place for it to stay, but now it's Greg who refuses to let him leave. Nature has the final say in the matter, however.