Basically, Moomin, a fantasy creature, finds out a comet is on its way to Moominvalley and he sets out on a journey to The Lonely Mountains to ask the scientists at the observatory when it's coming and what he can do about it. On the way he has a bunch of fun adventures escaping dangers such as whirlpools, hungry eagles, bushes, and crocs (apparently you merely have to throw a pair of woolly trousers to fend off a herd of crocs). Oh and he gets a crush on the Snork maiden (that's her pinning a star on Moomin on the cover).
This is really quite an absurd story told in a very matter-of-fact manner. It's silly in the best possible way - a juxtaposition of inventive situations and bizzare characters that never fails to get you giggling and shaking your head. My favorite character might have been the philosopher Muskrat who comes to live with the Moomins because their bridge-building activities destroyed his house - but maybe that's just because I love the drawing of him getting drenched in the rain.