The narrator, a woman in an unnamed Pacific Northwest city who goes by the pseudonym "Jane Smith," addresses a future reader from a position she assumes will be posthumous, stating she is there to show the reader how the world ends. Jane is a six-foot-tall, 230-pound former wrestler who works as a vulnerability analyst at a cybersecurity firm. She lives a comfortable but emotionally strained suburban life with her husband, a real estate agent, and their teenage daughter.
One cold winter morning, a barista hands Jane an envelope containing a key, an address, and the number 7. A note on the back states the sender is already gone and that Jane is on her own but "not alone." Jane rides to a remote storage facility called the "Imperial Storage Palace." Inside Unit 7, she finds a taxidermied hummingbird on a pedestal with the initials "R.S." and a note signed "Silvina" reading "Hummingbird ..... Salamander." The word "salamander" stirs memories of her childhood, when she and her older brother searched for the creatures along creeks on the family farm.
Jane hides the hummingbird at her gym and tasks her assistant, Allie, with researching Silvina. She learns that Silvina Vilcapampa was the 54-year-old daughter of Matias Vilcapampa, a wealthy Argentine industrialist. Silvina was an environmental activist arrested on conspiracy to commit murder charges related to the disappearance of wildlife traffickers but released for insufficient evidence. She recently died in a hit-and-run with minimal media coverage. Allie also identifies Ben Langer, an enforcer linked to Contila, an import-export company tied to wildlife trafficking.
Jane identifies the bird as a naiad hummingbird, a species presumed extinct since 2007, making it illegal contraband. She pries out its glass eyes and discovers two numbers etched in the sockets: 23 and 51. At a security conference in New York, she obtains encrypted phone equipment from a contact named "Jill" and visits Carlton Fusk, a Brooklyn taxidermist who pulls a gun on her and orders her to leave. At the conference bar, a man calling himself "Jack"—a stranger who appears to have singled her out deliberately—approaches with hawk-like intensity, gives her a false hotel room number, and vanishes. Jane connects the numbers to an apartment registered under the fictitious name "Alexander Humboldt."
Threats converge at home. Jane's husband discovers evidence of a watcher in the woods behind their house. Her colleague Larry is run over in the company parking garage after being questioned about Silvina. The hummingbird is stolen from her gym locker. A man follows Jane and warns her to stop investigating; she beats him badly. Texts from an anonymous figure calling himself "Hellbender," after the giant salamander species, begin arriving, demonstrating intimate knowledge of Jane's investigation and home life. He leaves a gun at the surveillance spot in Jane's yard and claims to be protecting her.
Jane breaks into Silvina's apartment and finds three eyeless taxidermied foxes and a small black journal buried in a planter. Allie delivers a declassified report revealing Silvina was under investigation for bioterrorism, then quits, accusing Jane of endangering the company. Jane's daughter confronts her about the gun. Jane travels to Unitopia, a half-abandoned island commune that Silvina once funded, and confronts Ronnie Simpson, the taxidermist whose initials mark the hummingbird's pedestal, at gunpoint. Ronnie reveals the location of a Vilcapampa-owned warehouse of taxidermy contraband, then attacks Jane and escapes. Jane is fired after the Vilcapampa family intervenes with her employer.
Jane drives to the warehouse at night, warns her family to flee, and texts Hellbender the address. Inside, she discovers an enormous mound of contraband animal remains. Langer and his men open fire; Jane takes bullet wounds in her thigh and arm before a fire breaks out and she loses consciousness. She wakes tied to a chair in Silvina's apartment. "Hillman," the man she beat earlier, forces her into a video conference with Vilcapampa Senior, who demands to know about Silvina's "final project." Hillman burns the taxidermied salamander before her. Jane bull-rushes Hillman, hurls herself off the third-floor balcony clutching the smoldering salamander, and escapes.
In a flashback, Jane reveals her traumatic family history. Her grandfather, "Shot," was an abusive man who terrorized the household. Her brother Ned was kind and introspective, transforming their harsh landscape through stories of mythic giant salamanders. Ned died by a creek one night, officially ruled a drowning. Believing Shot killed Ned, Jane pushed her grandfather off a barn roof two years later, killing him.
Months after her escape, Jane lives under a false name on a derelict houseboat, working as a private detective. She identifies the burnt salamander as a giant subspecies of an extinct road newt. She connects Hellbender to "Jack" through his use of the name "Jill" and renames him "Hellmouth." A case she takes turns into a deadly trap; she narrowly survives a shooting and finds Hillman dead in a motel room. His real name is Roger Simpson, Ronnie's brother. A coded message in his belongings points Jane to her family farm.
At the farm, Jane's father reveals that Silvina's family once lived on the ridge above and that Ned did illegal work for them, poaching salamanders and serving as a courier. He tells Jane that Ned was not the idealized figure she remembers. He hands her a letter Silvina left before dying. Silvina reveals that Roger Simpson murdered Ned on orders from her father because Ned entered the Vilcapampa house and saw something forbidden. Silvina writes that she carried this guilt for decades and chose Jane specifically, needing her to make the full journey to understand what was at stake. Jane realizes her killing of Shot was based on a false assumption.
After a bitter confrontation with her estranged husband, who was directed to the farm by Hellmouth, Jane returns to the storage palace. The fluorescent light in Unit 7 still works despite the power being dead elsewhere. Behind the casing, she finds a keypad and enters the hummingbird's numbers. A hidden panel reveals stairs ascending into the mountain. On the stairs lies Ronnie Simpson's mummified body, clutching the stolen hummingbird. Jane reclaims the bird and climbs.
At the summit, Jane discovers Silvina's secret project: a bunker inside the mountain. She finds Silvina's body in a chair, dead for years, and realizes with horror that Silvina was still alive during an earlier search Jane and Hellmouth conducted on the mountaintop, watching them through hidden surveillance cameras. Behind a massive window lies a self-sustaining ark containing a creek, trees, birds, and, according to a species list, hummingbirds and salamanders. The walls hold DNA samples for future species revival.
Three syringes sit on a medical station. Silvina used one and died. Ronnie took the second and died. One remains. Documentation describes them as biological transformation agents derived from the salamander's defensive toxin and the hummingbird's alkaloids, designed to alter the recipient's genetic code and spread the change through contact. Beside the station rests Silvina's manifesto, a 2,000-page document in Spanish titled
Unitopia.
Jane decides to take the last syringe. She composes her account for release on a 100-year delay alongside the manifesto, planning to seal the entrance to protect the ark and inject herself on the hillside. Her final words ask the future reader what the world looks like, whether a hummingbird or salamander still exists, and whether there is still a "you." An appendix of intercepted documents reveals Hellmouth's likely real identity as Jim Lord, a former government operative. Field reports mention sightings of a mysterious "Woman in Flames," and flyers from "Friends of Silvina" invite followers to gather for her resurrection, suggesting the movement persists beyond Silvina's death.