The House of Hades by Rick Riordan is the fourth book in The Heroes of Olympus series. It follows Percy Jackson and Annabeth Chase as they struggle to escape from Tartarus, where they were trapped at the end of The Mark of Athena. With the help of the Titan Bob and a friendly giant, they navigate the deadly underworld to reach the Doors of Death, which they must close to stop monsters from resurrecting. Meanwhile, their friends—Jason, Piper, Leo, Frank, Hazel, and Nico—race across the mortal world to reach the other side of the Doors in Epirus, Greece, facing various trials and battling Gaea’s forces. Hazel learns to control the Mist, Frank steps into his leadership role, and Nico reveals his personal struggles. In the end, the demigods succeed in closing the Doors of Death, but Gaea’s awakening is imminent, setting up the final battle in The Blood of Olympus.