As a five-book fantasy-adventure series, Trials of Apollo is Rick Riordan’s third set in the Percy Jackson universe. The series functions on the premise that the gods of ancient Greek and Roman mythology are real and have moved Olympus to New York City, where they continue to sire half-mortal children. Their children are demigods, or heroes, who are vulnerable to monsters but are safe within the borders of two camps. Camp Half-Blood is designated for the Greek demigods on New York’s Long Island, and Camp Jupiter is designated for the Roman demigods in California. Each pentalogy in the Percy Jackson universe fulfills its own arc, but the three series also fit together, as each subsequent series draws on the previous ones for characters and plots.

The first series, Percy Jackson and the Olympians, follows Percy, the son of the ancient Greek sea god Poseidon and a mortal named Sally Jackson. At the age of 12, Percy discovers his immortal parentage and demigod status, travels to Camp Half-Blood, and undertakes a quest to save the Olympian pantheon. After he successfully defends Olympus, the gods offer him immortality, but he rejects it, opting instead to live a mortal life with Annabeth, his girlfriend, who is a daughter of Athena.