“Absolution” is mostly an Area X origin story; even so, for those unfamiliar with the trio, a backward glance into the future might help.

Under the umbrella of a CIA-like agency named Central, the Southern Reach is tasked with exploring and containing Area X, a quarantined coastal zone on the lush Forgotten Coast that’s home to an expanding mysterious force.

The Area X region warps time, space and identity. It’s separated from the outside world by a nearly invisible barrier called the Border. Of the Southern Reach missions that have entered the region, only a handful of survivors have returned.

A 12th expedition — a team of women representing different scientific disciplines — eventually makes its way to the Forgotten Coast lighthouse, an iconic tower that might be Area X’s energy source.

At some point, the keeper, Saul, mutates into a translucent creature known as the Crawler. Furthermore, some of the principal characters generate doppelgängers. (One becomes a whale-looking thing with “many glowing eyes that were also like flowers or sea anemones spread open…”)

Intruding into these convolutions is the Séance & Science Brigade, a shadowy band of militant psychics conducting questionable and catastrophic experiments.

As the trilogy ends, the Border comes down, and an intensified Area X absorbs the paranoid, chaotic headquarters of the Southern Reach.

Which brings the reader to “Absolution,” two espionage thrillers merged into one sci-fi/fantasy phantasmagoria.

The book’s first section is a prequel, in which neither Area X nor the Southern Reach exists yet. Jack Severance, Central’s sinister spy chief, has hauled Old Jim back into service, ordering him to unravel a supernatural mystery known as the “Dead Town Disaster,” a doomed expedition of two dozen biologists to the Forgotten Coast 20 years earlier.

In preparation for his on-site cold case inquiry, Old Jim pours over the agency’s archival material regarding the incident. The goal was simple enough: establish a base camp near an abandoned village — Dead Town — and pursue “general explorations.”

But the camp is soon overrun by large white rabbits with cameras strapped to their necks. Then they’re attacked by an “uncanny” stranger known as the Rogue, who runs through them like “an avenging angel.” The few biologists who escape are psychologically harmed “beyond repair.”

Old Jim’s mission directive is to sort out what happened at Dead Town, but, of course, there’s always the secret mission, or “seek mish,” which is to eliminate the Rogue, assuming he’s still hanging around.

Assorted spook house episodes and phantom battles ensue around the Forgotten Coast, climaxing in a showdown with the Rogue.

As this part of “Absolution” comes to a close, Old Jim conceals himself in the wilds of the Forgotten Coast, certain he’s being set up by Jack, who simply wants him to find the money that’s disappeared from the Dead Town project fund.

Is Area X an alien wormhole, as some have suggested, where events and creatures are projections of humanity’s unconscious fears? Or is it an accidentally-on-purpose eco-apocalypse created by the cryptic Séance & Science Brigade?

As one character puts it, “I just know the answers lie so far outside the box that there is no box.”

VanderMeer is a dedicated environmentalist, and it might be said that, across 15 novels, the intricate structures of his fiendishly whimsical story lines correspond to the random microscopic motions and mineral accretions of the natural world.

But there’s clarity of thought reinforcing his imaginative prose that might be attributed to the rigor of his nonfiction pursuits. His compendiums, “The Weird” and “The New Weird,” edited with Ann VanderMeer, trace these hybrid genres to the baffling haunt of Lovecraft, whom, they say, represents “the rise of the tentacle, a symbol of the modern Weird.”

Indeed, “Absolution’s” final chapters uncurl like a king-sized Cthulhu lashing. Set “One Year after the Border Comes Down,” the tale veers into hallucinatory bedlam, decorating its action yarn bravado with maximum obscenities.

Lowry, a suspect figure from earlier in the Southern Reach series, returns as part of a berserk mission to find the “off switch” for Active Area X, as it’s now known. Of course, there’s a secondary objective, which is to wrangle, or eliminate, Old Jim, assuming he’s still hanging around.

Lowry devours high-powered amphetamines, as well as experimental drugs that “destroy visions.” In this kaleidoscopic state of mind, he and his comrades must use rifles that have morphed into gars (the fierce, elongated fish) to destroy their doppelgängers and triple-gangers made of “goo” that spring up from the brush like Children of the Hydra.

As it stands, the complex achievement that is the Southern Reach series — now a quartet that covers generations of strange activity — leaves unexplained many things that are inexplicable, and that’s just the way it is.

As Jeff VanderMeer wrote in Weird Fiction Review, the Weird genre “doesn’t promise a return to normalcy. It says some things are beyond us, and in a sense it’s humble because it isn’t claiming an all-knowing status for humanity.”

FICTION

“Absolution”

by Jeff VanderMeer

MCD

464 pages, $30