The Hydra is the guardian of the Pearl of Persephone that is hidden in the replica of the Parthenon in Nashville.
Percy Jackson, Annabeth Chase, and Grover Underwood go to the replica of the Parthenon in order to steal the Pearl and they hide in the bathroom until after closing time. They encounter five janitors and Annabeth shoots tranquilizer arrows on them, then Percy uses his winged shoes to get the Pearl on the statue of Athena. However, the five janitors wake up early and turn into the Hydra to attack the three teenagers, who take refuge from its fire breath behind columns, but Percy drops the Pearl. The three try to fight the Hydra's heads and Percy decides to use the winged shoes to fly up, and he eventually manages to cut off each of the five heads. Unfortunately, ten more heads grow back right after. Annabeth tries to distract the Hydra by shooting arrows on it while Percy goes takes back the Pearl, but one of the Hydra's heads sees him and it goes after him. Before the Hydra could breaths its fire on him, Percy uses his powers to create a wave from the water of a nearby sink to protect him. He then takes the Pearl and flees with his two friends, but the Hydra follows them and breaths fire at them. However, Grover gets the idea to uncover Medusa's head and uses it to petrify the Hydra, stopping its fire and killing it for good.