How do you keep a secret you actually cannot hide? Twelve-year-old Pepe has to deal with this question, after he finds a little, ugly but kind of cute lindworm in Lake Tegernsee in Bavaria. Together with Fronny he is having the time of his life.

Their friendship, however, does not stay simple problem free: It does not remain invisible to the villagers, that there is a big and dangerous animal living in the lake.