After
Annabeth Chase and
Percy Jackson fall into
Tartarus at the end of
The Mark of Athena, the other five
demigods of the "
Prophecy of Seven" (
Frank Zhang,
Hazel Levesque,
Jason Grace,
Leo Valdez and
Piper McLean), aided by
Nico di Angelo and
Coach Hedge, prepare to go to Greece to find and close the
Doors of Death from the mortal world in order to prevent the monsters of
Gaea's army from continuously resurrecting. In
Bologna, the
Argo II is raided by the
Kerkopes; Leo goes after them to retrieve his stolen
Archimedes's sphere, and takes an agricultural book belonging to
Triptolemus and an
astrolabe made by
Odysseus as recompense. He also sends the Kerkopes to harass the Roman army massing at Camp Half-Blood. In Venice, Frank, Hazel and Nico retrieve barley cakes designed to protect them from poison in the
Necromanteion from Triptolemus. In exchange for the barley cakes, Frank has to get a replacement snake for Triptolemus’ chariot, which is powered by two snakes. After remembering that his father,
Mars, can turn enemies to snakes, Frank decides to battle all the
Katobleps (cow monsters) in exchange for a snake. He defeats all of the monsters and receives the blessing of Mars for his heroism, becoming physically stronger and gaining a snake for Triptolemus. During a later encounter with the bandit
Sciron and after a meeting with the goddess
Hecate, Hazel successfully learns to manipulate the Mist, a power that alters other people's reality layers by deceiving them. At Jason's behest, the demigods travel to
Split to visit the tomb of
Diocletian, retrieve his powerful scepter, and leave a note for
Reyna (who has been asked by Annabeth to find the Greek demigods). The god
Cupid, guardian of the scepter, refuses to relinquish it until Nico admits that he once had a crush on Percy Jackson. While sailing through the Adriatic Sea, the ship is attacked by
Khione and the
Boreads. Piper uses her powerful charmspeaking skills to defend the group. During the attack, Leo is transported to
Ogygia, where he falls in love with
Calypso. Although he leaves the island, Leo promises to return for her. While Leo is detained, the rest of the crew meets with
Notus, who helps Jason to realize that he has chosen to be a Greek rather than a Roman demigod, settling an internal conflict within himself. Jason later gives up his
praetorship to Frank in accordance with this decision. Arriving at the Necromanteion, the reunited crew is attacked by
Clytius,
Pasiphaë, and a group of their minions. Each of the demigods uses some aspect of their newly strengthened powers or identities to help defeat these monsters; for example, Hazel's new power and alliance with Hecate helps her defeat Pasiphaë and Clytius. Meanwhile, Percy and Annabeth travel through Tartarus to the other side of the Doors of Death, aided by the Titan
Iapetus, who now goes by "Bob" after
a previous encounter with Percy, the giant
Damasen, and a few other beings. As the other demigods fight in the world above, Percy and Annabeth's group reach the Doors of Death and fight the personification of Tartarus, eventually destroying the chains holding the Doors in place. Bob stays behind amongst hordes of angry monsters to defend the Doors while Percy and Annabeth escape and reunite with the other five demigods, who successfully close the Doors of Death. Reyna arrives on her dying
Pegasus and Annabeth tasks Reyna, a Roman, with bringing the
Athena Parthenos (using the
Argo IIs hold) back to Camp Half-Blood to appease both the Greeks and Romans, with Nico and Coach Hedge volunteering to accompany her.
Characters •
Annabeth Chase: A demigod daughter of
Athena, the Greek goddess of wisdom. Along with her boyfriend Percy, she is trapped in Tartarus during the novel. •
Frank Zhang: Son of
Mars, the Roman god of war. He struggles with the voices of his father and Ares fighting in his head during most of the novel. After defeating all the katoblepones in Venice, he receives the blessing of Mars and peace from the arguing voices. He is dating Hazel Levesque. During the battle against the monsters in the Doors of Death, Jason Grace relinquishes his role as praetor of Camp Jupiter to Frank so that he can be of high enough rank to command an army of ghosts. •
Hazel Levesque: Demigod daughter of
Pluto, the Roman god of the dead and riches. When
Argo II was prevented from crossing the Apennines by Ourae, she met with Hecate, who showed her a northern path to travel, with the condition that Hazel would learn to use the Mist. At the Doors of Death, she uses the Mist against Pasiphaë, recreating the
Labyrinth of Daedalus and throwing the sorceress into an imaginary abyss. She also defeats the giant Clytius with the help of Hecate. •
Jason Grace: Demigod son of
Jupiter, the Roman aspect of
Zeus. In an audition with
Auster, the Roman deity of the South Wind, he reveals his preference for Camp Half-Blood instead of Camp Jupiter, and the god forces him to commit to his choice. During the battle against the monsters in the Doors of Death, Jason is unable to control the Diocletian's scepter because of his choice. •
Leo Valdez: Son of
Hephaestus, the Greek god of fire and forges. After Khione and her siblings attack the
Argo II, trireme built by Leo, he falls into Ogygia – he manages to escape with the help of Calypso after they fall in love. At the Doors of Death, he helps Hazel fight Pasiphaë and opens the door for Percy and Annabeth to return to the mortal world. •
Nico di Angelo: Demigod son of
Hades, the Greek aspect of
Pluto. Confronted by Cupid, he admits he had a crush on Percy and asks Jason to keep his secret. In Epirus, Nico leads the group to the Doors of Death. He works with Frank to summon and control the dead Roman legion against the cluster of monsters from Clytius. He and Hazel also use their ability to travel in the shadows to bring the group safely to the surface. Later he offered to take Athena Parthenos back to Camp Half-Blood with Reyna and Coach Hedge. •
Percy Jackson: Demigod son of
Poseidon, who is also the main protagonist of the
Percy Jackson & the Olympians series. He falls into Tartarus with his girlfriend Annabeth. •
Piper McLean: Daughter of
Aphrodite. When Khione attacked
Argo II, she rescued the rest of the crew after reactivating Festus (Leo's automaton dragon integrated with the ship) with her charmspeaking, which burned
Calais and Zethes and allowed Piper to defeat the goddess. •
Reyna Avila Ramírez-Arellano: Demigod daughter of
Bellona, the Roman goddess of war, is the praetor of Camp Jupiter. After she and the
augur Octavian meet Rachel and
Grover Underwood in New York City, Rachel gives her a message from Annabeth stating that Reyna was the only person able to take Athena Parthenos back to Camp Half-Blood and prevent the war between the Greeks and Romans. She agrees and goes to Greece on her pegasus, Scipio, in search of the other demigods, leaving Octavian in power. Upon finding them, she accepts the help of Nico and Coach Hedge to take the statue back to the United States using shadow travel. •
Iapetus/Bob the Titan: The Titan of the West and the father of
Atlas, Iapetus was previously encountered by Percy, Annabeth, Nico and Thalia Grace in the short story
Percy Jackson and the Sword of Hades where he had his memory erased by Percy throwing him into the River Lethe. == Composition and marketing ==