One day Moomintroll notices that something strange has happened in the Moominvalley – 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!