As the book opens, Jason awakens on a school bus, unable to remember who or where he is, or anything about his past. He is sitting next to Piper McLean and Leo Valdez, who call him by name and say they are his girlfriend and best friend respectively. All three are part of a class field trip to the Grand Canyon, and after they arrive, a classmate Dylan turns into a Venti (Storm Spirit) and attacks the trio and their trip leader, Coach Gleeson Hedge. In the ensuing fight, Jason surprises everyone, including himself, when one of his coins turns into a sword which he uses to battle the storm spirits. Coach Hedge, who reveals himself to be a satyr during the fight, is taken captive by a fleeing spirit. After the battle, a flying chariot arrives to rescue the trio, but one of the people in it, Annabeth, is upset when she discovers that her missing boyfriend, Percy Jackson, is not there as she expected. Annabeth, seeking Percy, was told in a vision from the goddess Hera to look there for the "guy with one shoe", but this turns out to be Jason, who had a shoe destroyed during the fight. Jason, Piper, and Leo are told that they are demigods and are taken back to Camp Half-Blood where they meet other Greek demigod children like themselves. There, Leo is revealed as a son of Hephaestus, Piper as a daughter of Aphrodite and Jason as a son of Zeus, though Hera tells him he is her champion. Jason later discovers that he is the full brother of Zeus's demigod daughter Thalia Grace, who is a Hunter of Artemis. Shortly after they arrive, the three are given a quest to rescue Hera, who has been captured, and they set off. They soon discover that their enemies are working under orders from Gaea to overthrow the gods. During their quest, they encounter Thalia and the Hunters, who have been looking for Percy. Thalia and Jason reunite for the first time since Jason was captured at the age of two by Hera. On the way to Aeolus's castle, Jason, Leo and Piper become separated from Thalia, who promises to meet them at the Wolf House, the last place Thalia had seen Jason before this meeting. After being nearly apprehended by Aeolus, who is under Gaea's orders, the trio manage to escape thanks to Mellie, Aeolus's former assistant, and end up in San Francisco, thanks to the result of a dream Piper had with Aphrodite. After landing in San Francisco, the trio rush to Mount Diablo to fight the giant Enceladus, who captured Piper's father. They manage to kill the giant with the help of Zeus sending a thunderbolt and save Piper's father, after which they rush to the Wolf House to free Hera. Although the heroes and the Hunters save Hera, the king of the giants, Porphyrion, rises fully, Hera's energy was being used to raise him, and disappears into a hole in the Earth to escape Hera as she uses her power to destroy the Monsters around her. Jason's memory then starts returning, and he remembers that he is a hero from a Roman counterpart to Camp Half-Blood somewhere near San Francisco, and is the son of Jupiter, Zeus's Roman aspect. He realizes that Hera, also known as Juno, has switched him with Percy Jackson, who will be at the Roman camp with no memory of his life, in the hopes that the two camps would ultimately work together to fight the giants and defeat the goddess Gaea.
Jason Grace: A demigod son of Jupiter and the protagonist of the story. Jason suffers amnesia at the beginning of the book and is inclined to call the gods by their Roman names. He owns a coin that turns into a spear or a sword depending upon heads or tails, although it is destroyed during their quest. He is 15 years old and Thalia Grace's younger brother, although Thalia was born under the Greek aspect of Zeus while Jason was born under the Roman aspect, Jupiter. Jason was offered to Hera as a champion at the age of two, which later provoked Thalia to run away from home. Before the start of the book, he lived in the Roman...