As the witty murder-mystery series continues, Bonbon returns for a pyrotechnic display aboard ship—until a shocking demise puts a damper on the festivities.

Shenanigans are once again afoot in the town of Widdlington, and this time they're on board the Dreadnowt—a galleon that’s set to host a fireworks display for the town. Bonnie Montgomery and Grampa Banks are on board as guests, but just after sundown, a fog rolls in and a terrible cry is heard over the crash of the waves. Captain Valancourt stumbles out of the mist, dripping wet and covered in red welts the like of which a sea monster would leave behind, and is soon pronounced dead by the doctor on board. The crowd is shocked, and Bonnie's fishy-o-meter is off the charts. While Inspector Sands takes over as captain, Bonnie transforms into Montgomery Bonbon, and it’s not long before she’s laid anchor on her prime suspects. Can Montgomery Bonbon hoist his moustache and figure out which scallywag is responsible for the death of the ancient mariner?