Persephone calls Percy, Thalia and Nico, all the children of the Big Three, into the Underworld to retrieve Hades's new sword from a demigod spy who stole it. The problem is that Hades' keys, which let anyone out of or into the Underworld, are in it. Persephone gives them a flower to track the half-blood. When all of the petals fall off, the sword thief has made it out of the Underworld. Percy, Nico and Thalia must retrieve the sword before time runs out. They catch the thief, Ethan Nakamura, but all the petals fall off because Ethan has raised the Titan Iapetus. Percy flings Iapetus into the river Lethe, a river that makes one forget their identity, during a fight. Although Percy also falls in, he stays dry because of his powers, being the son of Poseidon. Percy tells Iapetus that he is Bob and Iapetus believes him and is now harmless. When they return to Hades and Persephone with the sword, Hades is very unhappy about the sword and leaves in anger, threatening Persephone never to disobey him again. They later find out that Persephone requested the sword against Hades's orders.