It is spring in the valley and the Moomins are ready for adventure Moomintroll and his friends Snufkin and Sniff find the Hobgoblin's top hat, all shiny and new and just waiting to be taken home. They soon realize that his is no ordinary hat; it can turn anything--or anyone--into something else

When Moomintroll learns that a comet will be passing by, he and his friend Sniff travel to the Observatory on the Lonely Mountains to consult the Professors. Along the way, they have many adventures, but the greatest adventure of all awaits them when they learn that the comet is headed straight for their beloved Moominvalley.