Percy, Annabeth, Grover, and Thalia reach Olympus as the sky bridge from the elevator dissolves behind them. Percy saves the injured Annabeth from falling, but the collapse leaves the group marooned on the mountain. They realize the link between Olympus and America is failing, meaning Olympus may end permanently if Kronos succeeds.

The group runs through a devastated Olympus, where Kronos has shattered temples, gardens, and shrines with his scythe. A collapsing statue of Hera nearly crushes Percy and Annabeth, but Thalia pushes them clear and is pinned under the statue instead. Thalia orders the others to continue, so Percy, Annabeth, and Grover press on toward the ruined palace.

Inside the throne room, Kronos stands in Luke's body with Ethan Nakamura nearby, the hearth nearly dead, and the Ophiotaurus hidden in its sphere. Percy provokes Kronos into changing his scythe into Backbiter, and Annabeth recognizes the prophecy's cursed blade. Percy fights Kronos while Annabeth duels Ethan and Grover plays magic music that fills the room with courage and causes grass and roots to grow.

After Hephaestus's throne shocks Kronos, Kronos orders Ethan to kill Percy by exploiting Percy's weak spot. Percy appeals to Ethan's belief in balance, arguing that Kronos only destroys and will not truly help the minor gods. Ethan turns on Kronos, but Ethan's blade shatters against the Titan, wounding Ethan; Kronos kills Ethan by breaking open the floor and sending him falling through the mountain.

Kronos shows Percy the worsening battle in Manhattan and the gods' struggle against Typhon. Just as Kronos gloats, Poseidon arrives through the Hudson with Tyson, Briares, and an army of Cyclopes, helping the Olympians chain, wound, and sink Typhon into Tartarus. Kronos, enraged, resumes the fight, disarms Percy, and nearly kills Annabeth, but Annabeth reaches Luke by reminding him of family, his promise, and his mother's vision.

Luke briefly regains control as Kronos begins transforming toward his true form. Annabeth urges Percy to understand the prophecy, and Percy realizes Luke—not Percy—is the hero whose soul the cursed blade must reap. Percy gives Annabeth's knife to Luke, who uses it to stab his own mortal weak spot, destroying Kronos's hold and melting Backbiter into the hearth. Luke dies after asking Annabeth whether she loved him, receiving Percy's promise that unclaimed and neglected demigods will not be forgotten, and the Olympians arrive to find the battle over as Percy asks for a shroud for the son of Hermes.