Picking up after the events of House of Hades, our heroes are divided into two groups.
The first group is made up of Jason, Piper, and Leo (along with Percy, Annabeth, Frank, and Hazel). They're on the Argo II and they need to make it to the Acropolis stat. That's where Gaea, the Earth Mother, is supposed to awaken, and they need to stop it. (Of course, as goes with prophecies, trying to stop it is totally going to make it happen.)
Our other group is made up of Nico and Reyna who, along with Coach Hedge, are trying to get the Athena Parthenos statue back to Camp Half-Blood as a symbol of truce between Greeks and Romans…and hopefully stop Octavian from launching an onslaught against the camp.
With Nico's handy dandy shadow-travel ability, they're able to zip between time zones with the statue in tow. They travel through Pompeii, Portugal, a cruise ship in the Atlantic, Puerto Rico, and South Carolina, all exotic locales in their own rights. Along the way, they fight hordes of undead and hordes of werewolves and are pursued by a psychopath named Orion.
In Puerto Rico, Reyna's home, Reyna reluctantly reunites with her sister, Hylla, who wants to use her as bait for Orion. Bad idea. Orion wipes out most of the Hunters of Artemis before Reyna can escape. (Don't worry, sis is fine.) Orion catches up with Reyna near Camp Half-Blood, and Reyna summons her inner strength to defeat him.
As Nico sabotages Octavian's efforts to attack Camp-Half Blood, Reyna delivers the statue just in time…in time for Gaea to awaken, that is. Wait, how did that happen? Let's go back to team one.
Jason, Annabeth, and Piper learn that the strait of Corinth is chock full of death, and they have to go around the entire Peloponnese in order to reach Athens. They're advised to seek out Nike, the goddess of victory, along the way. She mentions yet another prophecy: one of them…will…die. Le gasp! But she advises them to seek out the physician's cure, which can bring someone back to life.
They retrieve the ingredients on their journey. Along the way, Jason is impaled, Percy is poisoned, Annabeth gets really scared, and everyone does battle with a statue demanding to see their health insurance. With the cure in hand, they reach the Parthenon where, yep, they end up feeding into the prophecy. Percy and Annabeth are injured, and their blood awakens Gaea.
Way to go, guys.
Leo flies in on a giant mechanical dragon, hauls Gaea's dirty butt into the sky, and blows her up. He manages to self-administer the cure, which brings him back to life, and he flies to the island of Calypso, the girl he loves.
And—say it with us—everyone lives happily ever after.