The novel is set twenty years after monsters emerged from a parallel world through a dimensional tear, devastating civilization. Humans retreated to cramped, military-controlled coastal cities, while the vast interior of the United States became the Wastes: a lawless territory filled with monsters and abandoned ruins.
Danny Sullivan, a twenty-three-year-old low-ranking soldier, narrates the story. After growing up impoverished in New Louisiana and losing his mother to untreated cancer, Danny enlisted out of desperation. Six months into his service, he is deployed under Captain Hamish with a two-hundred-soldier unit tasked with capturing Wyn the Soul Eater, a monster who appears every three years during an event called the Wild Hunt to massacre humans. Danny knows he is cannon fodder: a distraction force meant to die while an elite squad captures Wyn from behind.
Wyn appears on a ridge as a tall, hooded figure with long black horns and blackened fingertips. The ambush collapses almost instantly. Within two minutes, every soldier is dead except Danny, who passes out from terror as Wyn approaches.
Danny wakes at the underground military base in Nebraska, where captured monsters are held in reinforced cells. Lieutenant Mallory informs Danny that Wyn has been asking for the soldier who survived and refuses to speak to any officer. Danny is ordered to enter Wyn's cell and extract intelligence, despite warnings that previous interrogators experienced severe psychological distress or died suddenly.
Over daily sessions, Danny and Wyn develop a transactional rapport: Wyn answers approved questions about the Hunt in exchange for Danny answering personal questions. Wyn reveals that seven Soul Eaters exist worldwide and that his cycle occurs every three years because that is how long it takes to become necessary again. He accuses the military of torturing captive monsters and demands to see Danny's face. Danny complies, and Wyn confirms he can stop all seven Hunts but chooses not to. He also addresses Danny by full name despite never having been told it.
Days later, Wyn summons Danny at night. His fingertips dissolve into smoke against the cell glass, the base plunges into darkness, and Danny loses consciousness as screaming erupts outside. He wakes in a motel room in the Wastes. Wyn explains he was never truly trapped: He can dissolve into black smoke and travel through vents, and he allowed himself to be captured to infiltrate the base and free the other imprisoned monsters. The freed creatures killed most of the base's personnel. Wyn gives Danny three choices: wait for the pursuing military, go alone into the Wastes, or travel with him. Unable to return to the military he despises and terrified of being alone, Danny chooses Wyn and destroys his tracking chip.
As they travel west, Danny first witnesses Wyn kill at an old farmhouse. The target's face is deeply wrong, as though an inhuman mask is deteriorating. Wyn absorbs a dark, oily substance from the creature's throat, leaving the body an empty husk. The other humans in the house are unharmed. Wyn reveals the truth: A parasitic species from his world can infect humans, take over their bodies, and reproduce. Left unchecked, they would overrun humanity. His task is to destroy these parasites, not murder random people. When Danny asks why Wyn spared him, Wyn admits Danny was the first human to treat him without contempt.
Over weeks of travel, they develop a close friendship. When the military catches up in Wyoming, they flee into a forest where a kerenis, a nearly unkillable predatory creature, attacks the pursuing soldiers. They steal a truck and escape. Danny's romantic feelings intensify. When Wyn reveals he has been secretly carrying supplies Danny had to leave behind weeks earlier, Danny confesses his desire. They share their first sexual encounter, though Wyn pretends nothing happened for days until Danny confronts him and Wyn admits he reciprocates.
They encounter Edin, a large, lavender-skinned monster who is Wyn's close friend of roughly five thousand years. The three travel to a parasite nest in Colorado, where a small parasite bites Danny's arm, injecting lethal venom. Wyn sucks the venom from Danny's body at risk to himself. During Danny's recovery at a Colorado ranch, he overhears Edin urging Wyn to send Danny to a raider camp, a human settlement in the Wastes, for his safety. Wyn refuses but sounds guilty. Danny also hears Wyn confide that he will not show his face because he fears Danny will find it repulsive.
Their relationship deepens. They share their first kiss, though Wyn insists Danny close his eyes. Near the Grand Canyon, Danny demands to see Wyn's face. Wyn lowers his hood, revealing mismatched eyes, one jet black and one pure white, a prominent brow, a flat bridgeless nose, sharp cheekbones, and long black hair. Danny finds him beautiful and kisses him. They make love with Danny seeing Wyn for the first time, and Wyn agrees to keep his hood down when they are alone.
Their happiness is shattered in the Utah desert when a military sniper shoots Wyn in the head, destroying the left side of his skull. Mallory and soldiers shackle Danny and transport him to a new base, where he is starved, beaten, and tortured over many days. Mallory brands and carves the word "DESERTER" into Danny's chest and cuts off his left little toe. Hamish shows Danny a photo of Wyn's body and announces plans to dissect him. Danny refuses to cooperate, and Hamish beats him unconscious.
Danny wakes in a motel room to find Wyn alive, his head still healing and his left horn reduced to a nub. Wyn regenerated from the wound over several days before coming for Danny. He found Danny being beaten in his cell, killed Hamish, and carried Danny to safety.
During a month of recovery, Danny has nightly nightmares. Wyn refuses sexual contact while Danny heals, and Danny interprets this distance as rejection. When hostile monsters break into their motel room, Danny fights back with a baseball bat before Wyn arrives and kills the attackers. Their conflict worsens until Wyn reveals imprinting, a biological ability through which his species forms an involuntary, permanent bond of mutual awareness with someone they feel strongly about. He imprinted on Danny unconsciously at the base. Wyn then announces that Edin is coming to escort Danny to a raider camp for his safety.
Danny is furious and threatens to leave alone. Wyn blocks the door in a panic. Danny voices what has been tormenting him: Wyn has not touched him in weeks, and Danny believes he is no longer wanted. Wyn admits he has felt undeserving of touching Danny after all the harm Danny has endured. Their argument erupts into a passionate encounter, after which Wyn whispers that he does not want Danny to go. Danny tells Wyn he is the happiest he has ever been, and Wyn responds that Danny is his and that he has been waiting for Danny for a long time.
The next morning, Danny tells Edin he will not go to any raider camp. Edin accepts. Danny admits to Edin that he loves Wyn, not realizing Wyn is listening. After Edin departs, Wyn confesses he loves Danny more than anything.
Fully reconciled, they set out north together. Wyn reveals he has been traveling to the coastal cities at night in smoke form to kill parasites while Danny sleeps. When Danny asks what will happen when Wyn finishes his task and would normally return to his world to hibernate, Wyn tells him he now has a reason to stay. They walk into the desert hand in hand. A brief epilogue from Wyn's perspective reveals that from the very first moment he saw Danny on the battlefield, an ancient instinct recognized Danny as the one Wyn had been waiting for.