In this companion book to Rick Riordan's best-selling Trials of Apollo series, bizarre things are happening throughout Camp Jupiter.

To find out why, readers can consult the pages of this camper's journal, which reveals the secrets of the Roman demigods' training ground.

It's the February after Gaea's defeat, and mysterious incidents are wreaking havoc throughout Camp Jupiter. If the Romans don't figure out who--or what--is behind the episodes soon, the Twelfth Legion could implode.

Suspicion falls on Claudia, the Fourth Cohort's newest probatio. After all, the mischief started shortly after she stumbled into camp. Plus, she's a daughter of Mercury, the god of thieves and tricksters.

To find out whether she's guilty or innocent, readers can delve into the pages of Claudia's personal journal.

Through her eyewitness accounts, they will visit the crime scenes--a row of seats in the Coliseum, an underground aqueduct tunnel, the strange staircase at the back of the principia--and see the bizarre events unfold. They'll be right alongside Claudia when she discovers a secret so ancient that not even the lares know about it . . . a secret that holds the key to Camp Jupiter's safety.