Since becoming mortal as punishment for a near-world ending mistake and reaching Camp Half-Blood alive, Apollo—aka Lester Papadopoulos—has been busy. Sung his way out of the den of the Myrmekes, giant killer ants? Check. Protect the gates of an ancient oracle? Check. Fire a Shakespeare-spouting, plague-ridden arrow into the ear of a bronze colossus so it will sneeze its head off and die instead of attacking the camp? Check. Punch the long-lost, recently returned demigod Leo Valdez in the stomach? Check and double check.

But Apollo’s trials aren’t over. He has received a prophecy—in limerick form, the worst kind—from ancient Grove of Dodona. The prophecy instructs him to locate a second long-forgotten oracle in “a cave blue and hollow.”

Those words send him flying on Festus, Leo’s bronze dragon, to Indianapolis. Leo and his new girlfriend Calypso, former immortal, are with him. They’ve just collapsed Festus into suitcase form when they’re attacked by a horde of ruthlessly polite monsters called blemmyae. They make their escape and follow a headless orange ghost to an underground hideout called the Waystation. They’re met by two former Hunters of Artemis who greet Apollo with distinct chilliness. Why they dislike him is beyond him.

And then things start to get really weird…

Three Roman emperors who by all calculations should have died centuries ago are, in fact, very much alive. Apollo met one of the so-called Triumvirate, the malicious, power-hungry Nero, in the Grove of Dodona. Nero, he learned to his horror, raised Meg. She’s terrified of him—or rather, his alter superego, the Beast—and yet he coerced her back to his side in the grove. That did not please Apollo. As much as Meg annoyed him, he had grown to feel affection for her. Knowing she was under the control of a monster like Nero was hard to bear.

Now he learns the identity of another member of the Triumvirate—Commodus, a bloodthirsty narcissist with a serious grudge against Apollo. (That, Apollo can understand. He did drown the emperor back in the day, after all.) The emperor has big plans for Indianapolis that involve reviving games of bloodshed and death.

Stopping Commodus, finding Meg, figuring out why the former Hunters dislike him, and locating the long-lost oracle in a blue and hollow cave… Apollo’s to-do list is getting longer by the moment.