After escaping Kronos at Mount Othrys, Percy, Annabeth, Rachel, and Nico run blindly through the Labyrinth until exhaustion forces them to stop. Rachel has guided them away from traps, but the group is shaken: Annabeth grieves over Luke, Nico worries Kronos will send monsters, and Percy explains that Ethan Nakamura's pledge allowed the last piece of Kronos's spirit to enter Luke's body.

Annabeth refuses to accept that Luke is truly gone, arguing that Rachel's hairbrush briefly stunned Kronos and seemed to bring Luke back. Percy and Annabeth argue over Annabeth's continued faith in Luke, while Rachel tries to calm them. Nico reminds everyone that staying still is dangerous, so the group prepares to keep moving toward New York.

Before they can leave, Percy finds Grover's crushed Rasta cap and fresh tracks from Tyson and Grover. Although Nico warns that Camp Half-Blood has little time, Annabeth insists they must find their friends. The tracks lead the group through a wet, treacherous tunnel to a large cavern, where Tyson is cradling an unconscious Grover beside an underground river.

Tyson explains that he and Grover fought through dangers, but Grover collapsed after saying they were close. Percy realizes Grover is overwhelmed by the nearby presence of Pan, as he was once before in New Mexico. After Grover wakes, the group crosses the icy river and enters a glowing crystal cave filled with plants, extinct animals, and the ancient satyr god Pan.

Pan greets Grover and reveals that he has been fading for centuries because the wild places he embodied have largely disappeared. Grover begs Pan to return and save nature, but Pan explains that no god can do that anymore. Instead, Pan charges Grover to tell the satyrs, dryads, and nature spirits that Pan is dead, so they will stop waiting and take responsibility for saving the wild themselves.

Pan gives personal words to Percy, Annabeth, Tyson, and Rachel, hinting at their future roles and struggles, then asks Grover to release him. Heartbroken, Grover accepts the task. Pan dissolves into white mist that enters the group, especially Grover, and the cave's miraculous life crumbles to dust, leaving Grover older, sadder, and determined to deliver the message: the great god Pan is dead.