Annabeth and Percy are reunited with their friends Jason, Piper, Leo, Hazel, and Frank aboard the Argo II. Their mission is to prevent the awakening of Gaea, an ancient Earth goddess who wants to destroy the gods and rule the world.
A major conflict arises between Greek and Roman demigods, making teamwork difficult. Meanwhile, Annabeth receives a dangerous quest from her mother Athena: follow the “Mark of Athena” and recover a stolen statue, which leads her into a hidden and deadly part of Rome.
As the group travels toward Rome, tensions grow, friendships are tested, and battles against monsters and giants intensify. The story ends with a dramatic and emotional turning point that sets up the next book in the series.