Liz, a young librarian, finds herself in Hell after being hit by a bus-confusing, since she doesn't think she's a particularly bad person. Hell isn't how she'd pictured it, either. It's a crumbling bureaucracy with barely enough budget to house and torment souls. Satan has left the place in the care of a manager named Brian and Liz accidentally ends up running the help desk with him. Liz makes some friends, particularly Ellie and Theo, souls who live in her building. Dennis, a cat-like demon and personal tormentor lives with her, screaming for food and pooping in her underwear drawer.
Liz also works alongside Hagatha, an enormous She-Devil who heads security in Admin. Liz and Brian begin a quiet romance. Liz starts to think her afterlife might not be so bad, until she stumbles on documents that suggest Hell was originally conceived as rehab, established by grants from a mysterious Mountain Group. After souls were "rehabbed", they were to be given tickets out of Hell. Then Ellie discovers that real estate deals Satan made when he was setting up Hell further complicate the situation. He's leasing the property from creatures of the Abyss, the dark heart of nothing in the middle of Hell. After they finally contact the Mountain Group, Sam, their case worker, arrives in Hell and announces that the facility will be closed unless rehab begins, and souls can get tickets out. Sam attempts to set up a case for refugee status for the inhabitants of Hell, including demons.
Brian begins receiving mysterious emails. Sinkholes are appearing all over Hell. Ominously, Satan announces that he's coming to Hell before reinforcements from the Mountain Group can arrive to help. Brian, worried that Satan will abuse Liz, sends her with Hagatha to investigate the mysterious happenings. On their road trip, Hagatha, Liz, and Dennis meet Joe and Melchior, demons who formerly worked at the Abyss. They conjecture the Abyss is threatening the very existence of Hell and everyone in it. Brian, Liz, Sam, Hagatha, Joe, and Melchior meet with the creatures of darkness. They are told that they have a miniscule sliver of time to evacuate Hell because it will soon become nothing. Any soul or demon left will be annihilated.
Satan shows up and makes it clear that he knew that Hell was shutting down and doesn't care. Before leaving, he makes a mysterious threat against Brian.
When they return to the Administration building, they find that Satan has cut off Hell from the rest of the Netherworld. Everyone must evacuate in hopelessly short amount of time without a properly working train system.
They come up with a risky plan but as they're leaving, Brian tells Liz he is not allowed to leave Hell. He confesses his love and Liz watches from the back of the train as Perdition City collapses around him.
