It is the year A.D. 176; two boys leave their home town of Karanis far up the Nile and sail downriver to Alexandria on the coast. Renny has hopes of becoming a sailor and Lampon the Mouse Catcher is on a mission to persuade his father to return home from the temple of Serapis. Renny has no money for travel until he discovers a precious gem in a cave and sells it to Theodotus the Thief Catcher. Renny and Lampon sail to Alexandria, but soon after their arrival in the city they come to suspect that Theodotus has been murdered by the runaway slave Hermon. The reward for Hermon's capture gives Lampon another motive for catching him, though Renny is not so sure that this is any of their business. They chase Hermon into the underground cisterns and hunt him through the dark chambers of the tombs. The pursuers become the pursued, and Renny and Lampon are captured and thrown into prison. Finally they all confront one another at the top of the Pharos, the famous lighthouse in the harbor of Alexandria.