Gaea, the Earth goddess, is rising and wants to destroy all humans. The gods are suffering from multiple personality disorders as their Roman and Greek counterparts within each of them cannot agree with each other. Gaea cannot rise unless the blood of some demigods is dripped on the soil. Traps are set to make this happen. Meanwhile the Roman camp is attacking the Greek camp and the demigods must stop their destruction first before halting Gaea. The prophecy says that one of the demigods will die. The heroes know that they need the help of the gods but are having a hard time getting their cooperation as they war within themselves. When Gaea creates a trap the demigods must split up to do the impossible of saving the world.
Jason, Piper, Leo, Nico, and Reyna are the narrators of this story. They are trying to figure out their destinies as Greek or Roman heroes and figure out their fatal flaws or physical weaknesses. Greek tragedies have heroes that have misfortunes happen to them because of some error in judgement. Riordan pokes fun at this concept as the demigods banter with the gods. The demigods don't usually make errors and Riordan seems to be following the Greek comedy more than any tragedy. The heroes find happiness at the end in one way or another. If you have liked the series so far, you won't be disappointed with this one.