The seven demigods of the Prophecy of Seven—Percy Jackson, Annabeth Chase, Jason Grace, Leo Valdez, Piper McLean, Hazel Levesque, and Frank Zhang—go on their final adventure to defeat Gaea/Terra while Nico di Angelo, Reyna Avila Ramírez-Arellano, and Coach Gleeson Hedge attempt to bring the Athena Parthenos to Camp Half-Blood in order to prevent a war between the Roman and Greek demigods.

[3] The novel is narrated in the third-person, alternating between the points of view of Jason, Piper, Leo, Reyna, and Nico, making it the first time in the series that someone other than one of the seven demigods of the prophecy is the viewpoint character.

[2] After the events of the previous novel, Jason, Piper, and Annabeth disguise themselves with Hazel's Mist to infiltrate Odysseus' home in Ithaca, where Gaea's resurrected souls are gathering.

When Michael Varus, one of the souls, recognizes the demigods, he forces Jason to confront his mother's insane spirit, and severely wounds him.

As Jason recuperates, the three demigods use Odysseus' marriage bed to summon Juno, who tells them to seek Nike and find Artemis and Apollo, both of whom have been banished by Zeus.

At Olympia, Percy, Leo, Hazel, and Frank are forced to participate in Nike's deadly Olympic Games, but eventually captures her.

Reyna reveals that she killed her father's insane spirit to Nico and Hedge, and Bryce Lawrence, a legacy who was exiled from New Rome but recently reinstated, overhears.

Meanwhile, the demigods head to the Acropolis and battle the army of giants, whom they kill with the help of the gods in their Greek forms, their split personalities healed by the return of Athena Parthenos.

Meanwhile, Leo is resurrected by Festus with the physician's cure and arrives at Ogygia using an astrolabe created by Odysseus to pick up Calypso, fulfilling his vow to come back and rescue her.