A family weekend vacation to a casino turns into an ordeal in the desert for Hal, Reese and Malcolm, who wander onto an artillery range. Francis sneaks home, but has no one to play with.

Hal and Lois heads off to a convention while the boys stays home for the babysitter. Malcolm, Reese and Dewey assumed it's just another old woman, their parents hired to watch them. They decide to do a series of pranks on their babysitter, which includes the covered up sewer tank with a blanket and a book for the old woman sit on. To their surprise, their babysitter is a hot, teen-aged girl named Patty. She is sweet to them, but also bitter because she was rejected by Francis, a long time ago, because she was obese. Now the boys compete against each other to see who will prove their eldest brother wrong in his rejection and sleep with her without sex. Meanwhile at the convention, Lois tries in vain to stop the fight between Hal and Jack, who stole his idea.

The night shift at Lucky Aide is held hostage by two robbers, since Craig does not want to give up the combination for the store's safe. Francis tries to help a heartbroken friend at the academy, while Hal and the boys fight nature at home.

New neighbors, who turn out to be even more awful than Malcolm's family, move in next door; the Marlin Academy cadets prepare for a visit from Oliver North.

New monkey business occupies Reese, now a self-appointed law enforcer; a capuchin monkey has it in for Lois's co-worker, Craig.

Brotherly love is lost on Malcolm and Reese, now out to sabotage his sibling's birthday in devilish strikes met with crafty counterattacks.