Set in the early 1930s at a country house in Surrey. Witton Park is owned by crooked, multi-millionaire, oil speculator Walter Sinnet and he lives there with his wife Blanche and their two adult children Harry and Rose. Harry is a good-natured bookworm who spends most of his time in the library, and has nothing in common with his father who wants him to work in the family company. His sister Rose, fun but feisty, wants some excitement in her life. Walter has become a multi-millionaire by swindling the investors in his company and has a vast lake and underwater folly engineered and put onto his property. People can sit in the folly and look through the windows into the lake and watch the fish swim past. The Sinnet family have a weekend house party and amongst them are Joseph Brewer and his wife Florence who are a couple in need of money who are desperate to invest in Walter's business. Another guest is Hattie Abberton, who has been invited by Blanche and has a big secret to hide.Other guests include grieving widow Cordelia Brown, who is Blanche Sinnet's niece, jewellery expert George Brown and Blanche's childhood friend Aubrey Sapping, who is in love with her.That night a storm happens that is so severe that a large tree falls down over the entrance gates making it impossible for anyone to leave the estate. The next morning Florence Brewer's jewellery has disappeared and a dead body is found floating in the lake facing down through the folly's glass roof. Inspector Marcus Thomas and his son James, who has a day off school because of the storm, soon arrive at the property. Along with bumbling sidekick Constable Turner, the three set out to solve the mystery.