What do you do when your best friend is a rat called Halvorsen, and all the other kids think rats are disgusting? Svein and his buddy, Dan, are on their own until the new girl in class, Melissa, suggests they should participate with their rats in the big annual pet contest. But Halvorsen runs away, and the papers claim that both the hospital and schools have to be closed down because of a rat invasion. If Mum and dad find out it is all because of Svein and Halvorsen, not only the contest will be at stake. Svein also risks losing Halvorsen!