The most troublesome thing about being a detective is that it is almost impossible to go on holiday. When Bonnie Montgomery finally gets a break from cracking cases as her alter-ego, Montgomery Bonbon, something terribly fishy happens the moment she and Grampa Banks arrive on Odde Island for half-term and it's not just the smell of the harbour. There's been a mysterious death at Leerie Lighthouse, and what everyone assumed was an accident is setting Bonbon's moustache all a-quiver: a sure sign that something is Highly Suspicious. When a second death takes place on the other side of the island, Bonnie is sure the two are connected. But can she solve this case alone, or will she need double the detectives to solve double the murder?