Percy, Annabeth, and Grover enter DOA Recording Studios and find a lobby full of waiting dead spirits. At the security desk they meet Charon, who initially refuses them passage after recognizing that Percy is alive. Percy realizes Charon is bitter about his low pay and uses the gold drachmas from Procrustes's stash to bribe him, promising to mention a raise to Hades. Because Percy offers enough money and flattery, Charon agrees to ferry all three of them into the Underworld.

During the descent, the elevator shifts into Charon's boat on the River Styx, and the trio sees the river as a filthy channel of discarded mortal hopes and waste. The approach to the Underworld frightens Percy, especially as the dead around him and Charon himself appear more skeletal and inhuman. On the far shore, Charon leaves them at the entrance and they see the Underworld organized like a grim security checkpoint, with separate lines for judgment and for the ordinary dead bound for the Asphodel Fields.

As the trio studies the lines, they notice a corrupt televangelist being pulled aside for special punishment, which helps explain how judgment works for the wicked. They then finally see Cerberus, the enormous three-headed dog guarding the gate. Percy tries to distract Cerberus with a stick, hoping to treat the monster like a normal dog, but the plan fails and Grover translates that Cerberus is close to attacking them.

Annabeth saves the situation by pulling out a red rubber ball and using commands she learned from childhood obedience training. Cerberus responds immediately, sits, fetches, and becomes attached to Annabeth because she gives him the attention and play he rarely receives. While Annabeth keeps Cerberus occupied, Percy and Grover pass beneath the dog and through the EZ DEATH gate, then Annabeth reluctantly abandons the game and slips through after them.

The escape succeeds only partially. The magic detector and gate alarms identify the trio as living intruders, causing Cerberus to bark and Underworld security ghouls to begin searching for them while calling for the Furies. Hiding inside a rotten black tree, Percy reflects that both his failed stick idea and Annabeth's success came from the same truth: even monsters in the Underworld can be moved by simple kindness and attention.