Demigod Percy Jackson, still with no memory, and his new friends from Camp Jupiter, Hazel and Frank, go on a quest to free Death, but their bigger task is to unite the Greek and Roman camps so that the Prophecy of Seven can be fulfilled.

Though the Greek and Roman crewmembers of the Argo II have made progress in their many quests, they still seem no closer to defeating the earth mother, Gaea. Her giants have risen -- all of them -- and they're stronger than ever. They must be stopped before the Feast of Spes, when Gaea plans to have two demigods sacrificed in Athens. She needs their blood -- the blood of Olympus -- in order to wake. The demigods are having more frequent visions of...

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.

The Greek and Roman demigods will have to cooperate in order to defeat the giants released by the Earth Mother, Gaea. Then they will have to sail together to the ancient land--Greece itself--to find the Doors of Death.