Prologue

Disclaimer: If you have not read the first two stories, SPOILER ALERT!

The series starts a few days after the Second Gigantomachy. Leo is captured en route to Camp Half-Blood by a son of Hecate named Gaius and forced by means to do his dirty work. Percy is killed by Leo in a surprise attack, leaving the other demigods thirsting for revenge. Over the course of two days, they manage to track Gaius down, where they find him near Rio de Janeiro with his followers in two ships, a cruise ship and the Argo III, an upgraded version of the Argo II.

In the ensuing battle, the remaining Seven are captured and forced to watch as Gaius destroys the statue of Christ the Redeemer in Rio, blaming their world for it. He casts a bound Jason into the water as punishment for destroying the statue. Just as he is about to drown, Jason is saved by a revived Percy. While Percy distracts Leo, the remaining Seven manage to subdue Gaius, and he is killed while trying to escape by Annabeth throwing his own Mist-Manipulating knife at him. Unknown to the demigods, this was part of Gaius' plan, by becoming part of the Mist, he can destroy it, plunging the Olympians' world into a battle for survival.

During Percy's battle with Leo, the two tumble into Tartarus through the hole created by the destruction of the statue. Percy manages to break Gaius' hold on Leo, but the two have no way out. They are rescued from attacking monsters by Bob and Damasen, who were resurrected after they managed to make Tartarus enter his divine form. However, all other monsters to ever live were also resurrected, leading to their old enemies resurfacing. Percy, Bob and Leo are captured by Kronos and Porphyrion, and are forced to undergo heinous tortures.

In the real world, the demigods have taken over the Argo III and are returning to Camp when they find out that it has been mined and is rigged to blow soon. They rush back and manage to evacuate the camp minutes before it blows. They are joined en route by six of the Hunters of Artemis and the goddess herself, who help them fend off an ambush by a resurrected Orion.

Artemis and Chiron reveal that there is an exit from Tartarus in the Lake Avernus in Naples, information known also to Percy and Leo through Damasen. Annabeth and the Hunters of Artemis depart on Festus for Naples, but the Argo II and the remaining demigods continue to Camp Jupiter, where there is a threat of attack by Gaius' mortal soldiers.

Gaius reveals himself to Hazel when everyone is asleep, mocking her and telling her his plans for destroying New Rome and the Mist. Piper has visions of Percy, Annabeth and Leo dying at Lake Avernus because the other demigods were not there to help them, and forces Chiron to turn around and follow them. Chiron activates the 'jet mode' on the Argo III, and they link up with Annabeth and her group before continuing on to Naples.

Meanwhile, after three weeks (Tartarus time) of torture, Kronos sets Leo and Percy free, and brings the Iapetus side of Bob back, making him an evil Titan again. As the two leave the fortress, they are pursued by hellhounds, but rescued by Damasen. They make for the Acheron, only to find that they have been followed by Iapetus and Porphyrion. In the ensuing battle back on Earth, Iapetus is killed by Leo and Damasen by Porphyrion. Percy unlocks his Earth-shaker powers due to rage, and kills Porphyrion after torturing him, scaring Leo. The two are rescued from an army of dracanae and hellhounds by the arrival of the Argo III.