When spring comes to Moominvalley, Moomin and his family come out of their winter hibernation. Up in Lonely Mountains, Moomin, Snufkin, Sniff, and Little My find a large top hat. When they bring it home, they realise that it is a magic hat.

The magical hat that the Moomins tried to get rid of, reappears on a sandbank in the river, which is now filled with raspberry juice. Moomin hides the hat in the cellar, but Little My drops a piece of string into the hat and something odd happens.

Moomin and friends encounter Hemulen, who is sad because his stamp collection is finally complete and he has nothing more to collect. He tells them about a wrecked sailboat on the beach. They start repairing the boat, hoping to sail the seven seas.

The Moominfamily and their friends set out on an adventure in their new sailboat and they discover a seemingly uninhabited island.

Moomin and his friends and family spend the night at the Hattifatteners Island and goes on a treasure hunt.

One day, two tiny guests arrive at Moominhouse. They bring a suitcase, so Moominpappa knows they have come to stay and builds a bed. Everybody is dying to know what is in the guarded suitcase. The terrible Groke knows and she is coming to get it.