As Arthur prepares for his first sleepover, his sister D.W. and all his friends are talking about the UFOs that have been sighted in the area. However, Arthur, Buster and the Brain don't let the rumours interfere with their plans for a backyard adventure in Arthur's tent.

In typical fashion, D.W. waits until the boys quiet down for the night and launches her own 'alien' invasion, and Arthur thinks of a way to beat her at her own game.