One day Moomintroll notices that something strange has happened in the Moomin Valley - everything is grey; not just the sky and the river, but the trees, the ground, the Moominhouse, too! Moomintroll runs to ask the well-read Muskrat if he knows what is happening. Muskrat declares that this is the way things tend to look like before the earth faces an awful fate coming from the sky... With the help of his father, Moominpappa, Moomintroll and his worried friends build the best raft ever and start on a challenging journey to the observatory. The wise professors there must know what the threat from space is all about! The trek is not easy. The brave entourage, Moomintroll, Sniff and Snufkin face many obstacles, yet also meet new and old friends. But they make it to the observatory: the professor calculates that a ferocious comet will reach the Earth in four days, four hours, four minutes and 44 seconds. Maybe 4 seconds later. It's time to go back home as fast as they can - surviving an unexpected number of peculiar creatures and dangers on the way. Will they get everybody safe in time?