Slinoor captains one of three ships- the Squid- carrying gifts for a king in a land called Lankhmar. With him are the Mouser and Fafhrd who are meant to protect the ship as it sails. The two guards expect it to be easy, until Slinoor informs them that two other ships tried and failed to make the same trip; their remains were mysterious found floating in the ocean along with a dead crew. However the Mouser and Fafhrd are more unsettled by their cargo- about a dozen white mice who are smart enough to be trained to behave like men- they can fence, hold cups to drink out of, and so on. Their strangeness discomforts them but not to a great degree. Travelling closely with the rats is Demoiselle Hisvet, the daughter of Hisvin, the king of Lankhmar. Part way through the trip, the Squid is attacked by a dragon. A strange "demon" who speaks German and hops between different worlds rushes in riding another dragon to subdue it. The German's name is Karl Treuhertz, and he is roaming from world to world trying to find strange creatures to put in his zoo. He leaves, and soon after, seeing a mysterious black sailed ship, one of the other ships, the Clam, is lured away and destroyed. They see what looks like hoards of black rats swarming the Clam. The captain of another called the Shark boards the Squid and accuses Hisvet and her white rats of being responsible through witchcraft. The captain's name is Lukeen, and he demands that the white rats of the princess are counted. When they count only eleven, he demands that the gift rats are sunk and that the princess is locked away- in his memory there should have been twelve. Fafhrd, who had been infatuated with the princess, insited that there had always been eleven. Hisvet says the same. But Slinoor and the Mouser are certain there had been twelve. They argue about it, until Lukeen is incensed by Fafrhd and demands that they duel to see who is right. They duel and Fafrd wins, meaning the princess and her rats can stay. To thank him, the princess treats him to some food. The food is poisoned. The Mouser had some of the food as well, and the two of them ended up passed out. In this time, the Squid was again under attack, this time by black rats. Slinoor, convinced now that the princess is guilty, bursts into her room. But she and her maid already hid. He were unable to find them. Slinoor ties up the Mouser and Fafhrd so when they wake up they can't defend the princess. The princess reappears, dressed in leather at the same time the ship with the black sail comes beside the Squid. On comes Hisvin, commanding a rat army. He demands that Hisvet, who was revealed to be a were-rat, kills everyone onboard and sinks the Squid, but she refuses. The German demon appears and decides to help Slinoor and his crew. There is a fight and Hisvet, Hisvin, and the rest of the rats flee back onto Hisvin's boat. Hivet had shot the Mouser with a dart, but he was okay. The German, Slinoor, the Mouser, and Fafrhd sit around after the commotion and share stories.