One day, when the tide is exceptionally low, Moomin and the others go on an expedition to Circle Island, a round island with a lake in the middle which can be reached by foot. Sniff makes the find of his life in a cave and is subsequently lost in it. While the others look for him the tide comes back in, and they all have to stay overnight. None of them mind, though, because the Hattifatteners arrive, and they get to witness a quite remarkable spectacle.