As the Sea Rat pulls into port, the kids are excited to explore the town! That is, until Rotten Tooth makes them run errands the whole time. But they are determined to have fun, so the kids turn it into a spying mission. By the time they return to the deck, they have a whole list of suspicious activities to report. Rotten Tooth dismisses them, but then strange things begin to happen on the ship . . . could they have a stowaway aboard?