When Percy, Annabeth and Thalia respond to Grover's call for help, Grover tells them that he has found not 1 but 2 Half-Bloods. Siblings 12-year old Bianca and 10-year old Nico di Angelo are Half-Bloods though they don't know it yet. Although Grover has found them, he has realized that the Vice-Principal of the school where brother and sister are studying is actually a monster and about to attack them at any time. The rescue attempt goes awry at the last minute when Vice-Principal Thorn realizes what they are up to. Although Percy and gang manage to rescue the new Half-Bloods with the goddess Artemis
's and her Hunters' help, Annabeth is captured by Thorn and disappears. And when Bianca pledges herself to Artemis instead of joining the Half-Bloods, Annabeth's sacrifice seems even more fruitless.
After listening to Percy and Bianca's description of their encounter with Thorn, Artemis becomes convinced that a great threat to Olympus
is awakening and that she alone must go on a hunt for an ancient monster. Although she sends her Hunters back with the Half-Bloods to Camp Half-Blood, both Percy and Artemis's lieutenant Zoe Nightshade soon have dreams which convinces them that Annabeth and Artemis are the prisoners of Kronos's General.
A decision is taken for a group of 5 people, 3 Hunters and 2 Camp Half-Blood campers to go on a quest to rescue them. Percy however is excluded as the Hunters
are adamant that no boy joins them. In addition, Chiron
considers it best that Percy and Thalia do not go on the quest together as they each do not seem to be able to cooperate.
As usual Percy of course chooses to disobey Chiron and Dionysus
's orders. Percy promises Nico that he will protect Bianca while on the quest and secretly follows the group sent on the quest. When he finds the group being led into a trap, Percy helps them to escape. Due to this, Zoe accepts Percy into their group grudgingly. The Oracle
had predicted that a group of 5 would go on the quest, but one of the Hunters had been unexpectedly poisoned before the quest and was not able to join them. With Percy, the group have the necessary 5 predicted. As their group race to find Artemis and Annabeth while evading monsters, they are given unexpected aid by Apollo
In the end, although they successfully rescue Artemis and Annabeth, 2 of their number are sacrificed. Zoe is killed by her father, Atlas who is also the General of Kronos' army while Bianca disappears while helping the group escape Talos, one of Hephaestus
's defective creations.
Fearing a prophecy which foretells of a hero, a child of either Zeus
who may bring about the destruction of the Western civilization when reaching the age of 16, a few of the gods propose killing off both Thalia and Percy despite their recent heroic deeds. Thalia who would turn 16 the next day chooses to accept Artemis's offer to join her Hunters as her new lieutenant. Thus, as an immortal she will never reach the age of 16. In turn, Poseidon vouches for Percy and with the support of the majority of the gods Percy avoids death.
Returning to Camp Half-Blood, Percy however finds another candidate for the prophecy. When Nico lashes out at Percy upon hearing of his sister's death, Percy realizes that Bianca and Nico are Hades's children. Percy enlists Annabeth and Grover's help to find Nico when he disappears but are unable to discover where he went. Feeling guilty for failing to keep his promise to Nico, Percy makes Annabeth and Grover promise not to reveal the truth of Nico's identity to anyone. Percy vows to ensure that he is the subject of the prophecy when he turns 16 in 2 years time.