Lucy, Max, Charlie and Joe aren't looking for new friends – they have too many of their own problems to worry about. But when the four of them, and Charlie's faithful dog, Sherlock, spot a phantom figure on Southwold beach one winter's night, they are thrown together to unravel a mystery that none of them expected.

The deeper they dig, the bigger the adventure becomes – motorboats and tracking devices, bedroom breakouts and daring sea rescues are all in a day's work for the After-School Detective Club. But when their investigations lead them into trouble with the police, there is only one thing left to do – they must go undercover for a final showdown with a ruthless gang of smugglers who will stop at nothing to get what they want.