A treasure hunter wants to find a gold vein. But after several attempts, he realizes that there are no gold deposits in Moominvalley. He decides to move beyond the lonely mountains. But before he can put his plan into action, he comes down with a high fever. Moominpappa nurses him back to health. When he wants to move on after his recovery, he gives the Moomin family an old treasure map as a thank you. The Moomin Valley is depicted on it, and there is a mark at one point. Could there really be treasure hidden in the peaceful Moomin Valley? The Moomins want to put it to the test.