Exiting the container, they fight off the returning ravens and see the emperors’ enemy fleet of yachts approaching the Golden Gate Bridge. Meg grows a plant lattice to enable a quick descent from the tower, then collapses on the ground, exhausted by the effort. 

In their truck, Apollo confirms that Camp Jupiter’s communications are now restored when he uses a magical scroll to warn Frank Zhang, who is now at the Caldecott Tunnel, that the fleet is approaching. Trying to bypass the heavy traffic, Reyna swerves into an empty lane, but a eurynomos attacks the truck, causing the vehicle to lose control and crash.

Apollo wakes to find himself in a Target parking lot with Meg. Reyna is nearby with a broken leg. Lavinia arrives with a platoon of nature spirits, who move the injured to a hidden gully and treat their wounds. Refusing to acknowledge Reyna’s accusations that she has defected from Camp Jupiter, Lavinia claims that she is acting on orders and carrying out a secret “Plan L.” She tells Reyna, “When we all get back to camp, you’re going to thank me. You’ll tell everybody this was your idea” (313). She sends Reyna to sleep and has her group carry the unconscious praetor away, then prepares to attack the fleet from the sea. Left behind, Apollo and Meg see fires burning near Camp Jupiter.

Meg rents electric bikes, and she and Apollo ride toward the battle as the emperors’ forces overrun the Roman lines. Using a secret tunnel, the two emerge on the battlefield and reinforce the Third Cohort. A centurion directs them to Frank’s command post at the Caldecott Tunnel. There, Terminus warns that Tarquin is attacking New Rome through the sewers. Frank orders Hazel, Apollo, and Meg to Temple Hill to perform the summoning ritual while he holds the tunnel.

At Temple Hill, Tyson and Ella are ready for the ritual to begin. Ella chastises Apollo for being late. As Hazel and Meg leave to defend New Rome with the unicorns, Apollo despairs when he realizes that he can only choose a single god to summon. After dithering, he chooses to summon his sister, Diana, the goddess of the hunt. He completes the incantation, and Tyson and Ella add the ingredients. He drops the jar into the flames, releasing Harpocrates’s last breath, but Diana does not appear. Believing that the ritual has failed, Apollo sprints back to the front line to help Frank.

Apollo finds the Roman front shattered. Frank, who is now in bear form and bears several arrow wounds, leads the last few defenders against the emperors’ army. Commodus and Caligula arrive in a chariot pulled by maimed Pegasi, whose wings have been removed. Frank shifts to human form and challenges both emperors to single combat, stating that if they win, Camp Jupiter is theirs. Surprising himself, Apollo insists on joining Frank, making the proposition a two-on-two duel. A mercenary guarantees that there will no interference from outside forces. Frank orders his troops to fall back to New Rome, and then finds a spare moment to secretly tell Apollo, “You’re interfering with my plan. When I say ‘Time’s up,’ […] I want you to run away from me as fast as you can. That’s an order” (346).

Despite having been blinded in the last installment of the series, Commodus quickly disarms the weakened Apollo. Frank lures Caligula into the Caldecott Tunnel, and Apollo follows suit with Commodus, reasoning that the emperor will be less likely to track him amidst the tunnel’s confusing echoes. In the tunnel, a brief surge of divine strength allows Apollo to wound Commodus. Apollo then notices explosives lining the tunnel walls. Meanwhile, Frank takes bird form and pecks Caligula’s eye, then tells Apollo to cripple Commodus. Near the end of his strength, Apollo makes a supreme effort and stabs both of Commodus’s knees. 

Frank chokes with Caligula, who stabs him in the stomach. Frank yells “Time’s up!” to signal Apollo, then ignites his own life-tethered firewood, dedicating the sacrifice to Jason. The flames engulf both him and Caligula, igniting the fuses on the explosives. Grief-stricken to have witnessed the death of yet another hero, Apollo runs away as the tunnel explodes.