The Stupids attempt to discover what happens to the garbage that disappears from their lawn every week (they think it has been stolen) and accidentally save the world. The father, Stanley, goes undercover posing as a tree and unknowingly stumbles into an international arms-smuggling scheme. He believes that the garbage theft is perpetrated by his old enemy from the Post Office, "Sender". His absence worries his equally befuddled children, who go looking for him. Meanwhile he is chased by both the weapons smugglers and beings from outer space.