Siblings Julian and Dick Kirrin have been given a four-day weekend from their boarding school, coinciding with the mid-term break of their sister Anne and cousin George, so they arrange to go hiking together.

Julian decides they should hike to a ruined house called Two-Trees, located at Gloomy Water, a marshy lake higher on the moors.

The following day, Dirty Dick arrives at Gloomy Water with a woman called Maggie, to search for the loot and are annoyed to see the children there.

The next morning, they locate the Saucy Jane at the bottom of the lake, at which point Maggie and Dirty Dick appear in a boat but row back to shore after Julian says the children will be returning to school tomorrow.

The Five return on the raft near midnight, and Julian and Dick dive to the boat to haul up the loot, which is jewellery stolen from the Queen of Fallonia.