It’s time for Lasse and Maja to solve their twelfth case. The animals in the pet shop aren’t very well; they’re getting more and more tired, and now they don’t even want to eat. Could there be something wrong with their food? Valdemar, the owner of the pet shop, doesn’t know what to do. And it doesn’t help when the postman complains about dogs barking in the shop at night. That’s impossible, Valdemar doesn’t even have a dog! Lasse and Maja realise it’s time to start keeping an eye on the pet shop.