There’s a crime wave on the lake! All evidence points to three boatbuilders’ sons. They claim to be innocent but, as accusations mount, their families’ livelihood is threatened. It’s up to the three boys, and three other members of the Coot Club Bird Protection Society, to solve the mystery and clear their name.

Joe, Bill, and Pete live to sail in the Norfolk Broads but now they are under suspicion of setting boats adrift and stealing from ship builders. When the Ds (Dorothea and Dick) arrive, hoping to enjoy a holiday with their friends of the Coot Club, it seems to them that a plot is afoot. Someone is trying to drive Joe, Bill, and Pete away in disgrace for crimes they didn’t commit.