There are worse places to bring your girlfriend on a date than Tartarus. Oh, wait. No, there aren’t. Tartarus has poisonous air, liquid fire lakes, and land masses teeming with monsters, giants, and Titans. All in all, Percy Jackson wishes he and Annabeth Chase had fallen into any other pit. But at least they’re together. And alive . . . for now.

Their chances of survival tick up a notch when they meet up with a memory-challenged giant in a janitor’s uniform. Bob, his name tag reads, though Percy knows his real name is Iapetus because . . . well, he’s the reason Bob lost his memories. (A long story involving Hades, a sword, and the River Lethe.)

Bob knows Tartarus like the back of his gigantic hand, and he’s delighted to help Percy and Annabeth find their way around. His enthusiasm dims a bit, though, when they tell him they need to reach the Doors of Death and close them, fast. Even now, the evil beings of Tartarus are streaming through those open Doors to the land above. They are joining the unstoppable army of the primordial earth mother, Gaea, to help her destroy the world. They are unstoppable because unless the Doors are sealed, the monsters cannot die.

The Doors are in the very worst part of Tartarus. Getting there means evading monsters, many of whom ended up in Tartarus after Percy, Annabeth, and other Greek demigods killed them. Arachne the giant spider, three vampiric cheerleader empousai, the goddess of misery—they’d like nothing better than to take their revenge on the duo.
Just another day in the life of heroes seeking to save the world from total annihilation.

Meanwhile, Jason Grace, Hazel Levesque, and Frank Zhang—three Roman demigods—and Piper McLean, Leo Valdez, and Nico di Angelo—three Greek demigods—are desperately trying to keep their promise to Percy and Annabeth to reach their side of the Doors of Death. If they get there in time, they can open their Doors, rescue their friends, and save the world. Miss their connection with Percy and Annabeth, though . . . and it’s game over for them all.

Unfortunately, their Doors of Death are in the House of Hades, which is somewhere in Epirus, Greece. Getting there means crossing paths with many ancient and angry mythical beings—boulder-hurling monsters, a gigantic sea turtle, and a sharpshooter bandit with truly disgusting feet, among others—most of whom have chosen to side with Gaea against the Olympians. They’re dealt a blow when Leo vanishes for days, but luckily, he reappears when they need him most. Meanwhile, they get assists from a few gods, but only after they prove themselves worthy. Because trying to save the world isn’t proof enough.

The clock is ticking. Percy and Annabeth are dying. The six other demigods are struggling to keep their promise. Will they beat the odds again and succeed—or will unkillable monsters continue flooding from Tartarus to help Gaea overthrow the world?