“I could kid myself that I’m doing it to save the human race,” she says, “but honestly? I’m doing it for the money. The money I owe to the aquarium for their octopus.” But no one does something so dire merely for the money, as another volunteer observes. (“There were always easier ways of making a few quid.”) And, then, there’s that octopus, a creature Neffy came to know while employed at the aquarium, whose fate, we learn, is bound up with her present predicament — her desperate need of money, yes, but also the grief and guilt informing her decisions.

As those who swooned over the documentary “My Octopus Teacher” know, and as Neffy tells us, “It is possible to fall in love with an octopus.” Neffy, it seems, learned this at a tender age, snorkeling off a Greek island — where her father remained when Neffy and her mother returned to London, setting up a childhood split between the cool reality of England and the magical land of warmth and myth. Pursuing an octopus-adjacent career as a marine biologist, and then as an aquarist (one who manages an aquarium), she has discovered, with career-ending consequences, that caging or conducting experiments on octopi is too cruel for her to bear.

Much of this we learn from a series of mysterious letters, addressed to only “H,” that Neffy composes while confined to the facility where the vaccine test is being conducted — and through another device Fuller cleverly works into the increasingly complicated relations between the volunteers — at least the few who are left once things go disastrously awry. One of them, Leon, has developed a process for allowing people to relive their memories (“Revisiting,” he calls it — some sort of technological mumbo-jumbo that we might as well take on faith, because, after all, the world is ending), which works particularly well on Neffy. While under its spell, she inhabits past moments with her present understanding intact, an experience infused equally with elegiac joy and frustration.

Meanwhile the virus is mutating into an even more virulent variant, stranding the trial participants at the BioPharm facility, interrupting the proceedings, scattering the staff and leaving Neffy as the only one of the remaining five volunteers who has been given both the vaccine and the virus … and survived.

Having watched horrific scenes unfold on the streets below their windows, and having witnessed the collapse of the trial while Neffy hallucinated her way through the virus, the other four — a mix-and-match cast of 20-somethings who were never vaccinated, and so are still susceptible to the virus — clearly know something they’re keeping from her, even as they look to her, with her presumed immunity, to go out and procure supplies once their food and water run out. They also seem to have conceived a plan for her, the only successfully vaccinated person in the world, to begin producing immune children.

Imagine a “Lord of the Flies” where everyone on the island has opted in; or a “Breakfast Club” where anyone who leaves dies: Relationships will form and deform and character will win out. If you, like Neffy, were increasingly likely to be the last one standing, would you, like Neffy, be tenderhearted, kind, and resourceful enough to inspire hope for humanity at, literally, the last minute? It’s a neat trick that Fuller pulls off, weaving together so many familiar threads, from the post-pandemic storyline to the extremity-in-isolation scenario to the life story reconceived under duress, and yet coming up with a new and promising pattern — an authorial performance in keeping with her generous character.

Ellen Akins is the author of four novels and a collection of stories, “World Like a Knife.”

Tin House. 277 pp. $27.95