Uniting with Jason, Piper, and Leo after their dangerous quest, Percy, Hazel, and Frank wonder who will be chosen to fulfill the Prophecy of Seven, completing their group's number, and sail with them to an ancient land to find the mysterious Doors of Death.

Pia is a soft-spoken middle schooler whose life is turned upside down after the loss of her younger brother, followed by her parents' decision to move to a new town. In an effort to get her mind off of the troubles at home, Pia goes on a bus tour with a family friend, stopping at weird and wacky roadside attractions. The final destination: a mysterious underground lake. The locals say it has magical powers; Pia won't admit she believes in it, but...