It's a new school year, and Ava and Fia return to the Accademia on probation. Worse, their new headmaster is none other than Triton, Poseidon's domineering son, who transforms the campus into his own sea-themed domain.

As Fia struggles with her fear of water and the mystery behind her mother's suspicious death, Ava struggles to unlock the secrets of Fia’s lineage, including those of her ancestor Chimera.

With the help of their friends, a riddle from a Sphinx descendant, and the mythical Pegasus, Ava and Fia embark on a daring journey to Poseidon's golden palace to uncover the truth about Fia's mother. Along the way, they confront gods and reveal the real stories behind the so-called monsters. But Triton isn't forgiving, and it will take more than courage to right these ancient wrongs and escape with their lives.

Poseidon's Prisoner is the latest in Katherine Marsh's "unique and distinctly feminist fantasy series" (Publishers Weekly). The first book in the series is the award-winning Medusa, about which author and educator Colby Sharp said: "you won't be able to stop turning pages."