Al is on vacation when a human named Harold moves into the house he'd been using to operate an interplanetary doorway. Al makes Harold chocolate chip cookies in an effort to soften the news that not only do aliens exist, but they're also frequent visitors to planet Earth. Oh, and the house's previous owner (a cat) gave Al permission to set up the doorway in the basement. Harold takes this in stride until the children start to arrive and the monkeys attack.

Harold moves into a new house and comes home one day to find a strange man sitting in his kitchen. A strange, alien man, who has been using his formerly vacant house as an interplanetary doorway for extraterrestrial travelers.

The man goes by "Al." He works out of the basement. And he bakes a mean chocolate chip cookie. All things considered, Harold is willing to give him a chance.

What he doesn't realize is that accepting Al means adopting his family as well, which at last count stood at or around infinity. At least the kids have their names sewn into their backpacks. The adults' names are a lost cause.

With the help of the house's previous (feline) owner, some college students, a startling number of townspeople who also turn out to be aliens, and eventually Harold's own family, they do all right together. They manage to live on hot dogs, cereal, and cookies, and they only misplace one kid. The lights are always on when Harold comes home.

That is, until the monkeys attack.