Svein thinks his grandma's soul is in the process of being swallowed by mysterious aliens, and he mobilizes both his friends and his rat Halvorsen in an attempt to save her.

Svein's best friend is a fancy rat named Halvorsen who goes missing during a pet contest, leading to chaos with newspapers reporting a rat invasion. To avoid losing both Halvorsen and the competition, Svein must work to keep his parents unaware of the situation.