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 which turns out to be magical.

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.

When Hemulen tells Moomin and friends 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. They discover a seemingly uninhabited island.

Moomin spends the night at the Hattifattener's Island along with his friends and family. The next morning, they go on a treasure hunt.

When two tiny guests arrive at Moominhouse with a suitcase in tow, Moominpappa constructs a bed for them. Everyone is curious about the closely-guarded suitcase's contents.