Benjamin and Trap feel that Geronimo is over-worked and so they concoct a story to lure him away from his desk. It's a story about a sea-monster terrorizing the New Mouse City harbour. In an effort to prove to his cousin and nephew that there are no such things as sea-monsters, Geronimo goes along. Once at the harbour things take a turn for the worse. Our heroes find themselves held captive by the cruel Pirate Blackrat who is in need of a crew to help him find the lost treasure!