Snufkin Doesn't Come Back

Once again the very fine weather is boring Moominpappa to tears and this time he decides to take the whole family with him and sail for a lighthouse on a small island, not bigger than a fly-speck on the chart. Loaded down they set off and arrive next morning after a harrowing experience with a ghost-ship during the night. They meet a small boat with a man and a boy fishing, but when they get to the lighthouse it is empty, thick with dust and the beacon has gone out. What has happened?