Sur une colline, Moumine et ses amis trouvent un grand chapeau noir. Ils le rapportent fièrement à la maison, sans se douter que ce beau chapeau va provoquer une cascade d'événements rocambolesques dans toute la vallée. Parfois, le bonheur, c'est simple comme bonjour !
"When all the people of the forest started making the same secret sign (a star with a tail on it) Moomintroll, who was a thoughtful fellow, recognised it as a warning. So he and Sniff, the small animal, set out for the Observatory on the Lonely Mountains to ask the Professor if a comet was really on its way to destry the Earth. And on their way they met some interesting people, and had some narrow escapes"--Jacket.