The series begins with "The Lost Hero", which is followed by "The Son of Neptune", and then "The Mark of Athena". Some characters from the "Percy Jackson & the Olympians" series return, including Percy Jackson, Annabeth Chase, Clarisse, Chiron, other Camp Half-Blood campers and others. There are also some new characters. Some are Leo Valdez, son of Hephaestus, Jason Grace, son of Jupiter (Zeus as a Roman) and Piper McLean daughter of Aphrodite and movie star Tristan McLean who is kidnapped in "The Lost Hero".