Two months after the
defeat of Nero and Python, the
Oracle of Delphi,
Rachel Elizabeth Dare, provides the prophecy for the
demigods and
heroes,
Nico di Angelo and
Will Solace, the sons of the
Greek Gods Hades and
Apollo, respectively, as the boys follow the
prophecy on a
quest into the deep dark abyss of
Tartarus, underneath the
Greek underworld, to rescue an old friend of the boys, commonly known as
Bob, who is, in fact, the
Greek Titan Iapetus. Making their way into the Underworld through the Door of
Orpheus, which Nico and Percy had
previously used, the two demigods are immediately set upon by the demon of nightmares,
Epiales, who reveals that their mother is after Nico before Will manages to kill the demon using his newly-discovered power to emit light. Nico and Will seek the help of the troglodytes, who agree to lead them to a shortcut to Tartarus. But first, they pass the farm of Menoetes, who only agrees not to tell Hades about their quest in exchange for some stolen fruit from Persephone's garden. Although the heist is successful, Will encounters Persephone herself, who offers advice on his struggles to understand Nico, suggesting that Will embraces his own inner darkness rather than rejecting it, before gifting Will the stolen pomegranates. Pleased with their success, Menoetes reveals that Bob, while reforming from dying while helping Percy and Annabeth, was captured by the primordial goddess of night,
Nyx, who seeks to force Bob to go back to being Iapetus despite Bob's continuous rejection of his past. Nico reveals that he had encountered Nyx while traveling through Tartarus before
getting captured by the giants and she took an interest in him, realizing that Nyx is using Bob as bait to get at Nico. The trogs lead the demigods to the
River Acheron, where the nymph
Gorgyra agrees to help them in exchange for Nico and Will sharing their story with her. Using Gorgyra's
canoe, Nico and Will descend into Tartarus where the power of the river forces them to confront their insecurities and differences of opinion, particularly on the nature of the Underworld. Locating Bob's pet saber-toothed skeleton cat, Small Bob, the demigods discover that Percy and Annabeth's giant friend
Damasen is still reforming and can't help them before finally reaching Nyx's home, the Mansion of Night, where Bob is imprisoned in a continuous regeneration cycle. By combining their powers, Nico and Will free Bob before
Nyx arrives with
Nemesis,
Hypnos, a reformed
Epiales, and an army of monsters to cut off their escape. Nyx reveals that she has created
cacodemons out of manifestations of Nico's darkness and urges him to accept his darkness and stay in Tartarus, where the goddess believes he belongs. With the encouragement of Will and Bob, Nico fights back against Nyx's manipulations, embracing the fact that he is more than just his darkness, while Will accepts his own darkness to tap into Apollo's plague powers to fight Nyx. Taking control of the cacodemons, Nico decides to release them, symbolically letting go of his demons and fulfilling the part of the prophecy that he would have to leave something of equal value behind. Nemesis, Hypnos, and Epiales turn on Nyx and restrain her long enough for the group to get on the Acheron, which Nyx can't cross. To Nico's surprise, the cacodemons, which he renames
Cocoa Puffs, decide to join them of their own free will. As Bob sails the group back up the Acheron to the Underworld, Nico is visited in a dream by Hades and the spirits of his mother, Maria, and sister, Bianca. Hades reveals that he had sent Nico the prophecy so that he would rescue Bob, and arranged this dream. Maria and Bianca express their love and pride in Nico, as does Hades, who admits that Percy's actions have made him rethink his behavior towards his son, particularly when Hades realizes that the quest has risked Nico losing someone else that he loves. Hades, Maria, and Bianca urge Nico to enjoy his future with Will and find happiness. Returning to Camp Half-Blood, Bob declines an invitation to stay and decides to head west to figure out his future, looking forward to finally having the freedom to do so. Nico and Will adopt the Cocoa Puffs and contacts
Piper McLean and her new girlfriend Shel to discuss their shared grief over the loss of
Jason Grace. With a more hopeful outlook on life, Nico looks forward to what the future holds for him. ==Sequel==