It had been raining for days in Moominvalley, but now the clouds have cleared and it is a fine day for an adventure. Moomintroll is excited - he and Snorkmaiden and Little My, Moominpappa and Moominmamma, were all going to sail to Lonely Island to look for treasure! But by the time everyone is ready to leave, the sun had set and night had fallen. Would they ever be able to find any treasure just by the light of the moon?