Benjamin, Trap, and Thea are concerned that Geronimo Stilton works too hard as editor-in-chief at the Rodent's Gazette. To get him away from the office, they decide to investigate the mysterious three-headed brie-smelling monster that is roaming around the port of New Mouse City. But, things take a turn for the worse, as our heroes find themselves held trapped like rats on a ship... commandeered by the dreaded Captain Blackrat. Will it be a pirate's life for Geronimo and his relatives? Perhaps a cruise is the vacation he was needing!