Tracking the perilous journey of two lumberjacks lost in the wilderness, a desperate rescue team finds themselves on the edge of a cliff, nearly following their trusty hound over the precipice. In a harrowing twist of fate, two men take the plunge to retrieve the fallen dog but meet an untimely demise. Soon afterwards, the third team member hears chilling screams wafting through the still night air. As he descends to investigate, he discovers the horrific scene of his teammates lifeless, only to become a victim himself of an unknown malevolent creature lurking in the dark.
Dr. Robert Verne, played by Richard Dysart, has grown weary of the appalling conditions within the city's tenement housing, feeling his efforts lack any meaningful impact. In search of purpose, he takes on a position with the United States Environmental Protection Agency, tasked with conducting an extensive study on a contentious logging operation by the Androscoggin River in Maine. The loggers are enmeshed in a bitter conflict with the local American Indian tribe known as the "Opies," the native inhabitants of the land.
Accompanying Dr. Verne on this venture is his wife, Maggie, portrayed by Talia Shire. Anxious yet determined, she harbors a life-altering secret: she is pregnant but holds reservations regarding sharing this news as her husband is adamantly opposed to starting a family. Upon their arrival, the couple encounters Travis Nelson and his children, Paul and Kathleen, who are setting off on a wilderness expedition. Their arrival is followed by a meeting with Bethel Isely, the paper mill director and their designated host, who expresses concerns about a missing rescue team and points fingers at the Opies for the incident. However, the tribe has their own terrifying interpretation: it involves Katahdin, a vengeful forest spirit stirred into wrath by the logging activities, described by Isely as “larger than a dragon with the eyes of a cat.”
As Isely drives them towards their cabin, their journey is interrupted by an assembled group of Opies who block the path. An intense altercation arises between the formidable John Hawks, played by Armand Assante, the Opie leader, and Kelso, Isely's heavy who is armed with a chainsaw. With tensions escalating, Hawks finds himself vulnerable on the ground, the chainsaw perilously close to his throat, leading the Opies to relent and allow passage. The encounter leaves Verne and Maggie visibly shaken and concerned about the precarious state of affairs surrounding them.
While out fishing, Verne witnesses the brutal cycle of life as a giant salmon devours a duck. The couple's respite is shattered when a frenzied raccoon invades their cabin, an event that culminates in Verne killing the animal and sending off a tissue sample for analysis. The very next day, Hawks, along with his wife Ramona, portrayed by Victoria Racimo, approaches Dr. Verne to share their side, arguing that if he’s genuinely concerned about the environment, he must consider the people inhabiting it too. They reveal alarming truths of stillbirths, mental health crises, and birth defects tormenting their community, accentuating the environmental damage at play.
During their trek to the home of Hector M'Rai, Ramona’s grandfather, the couple hears firsthand accounts of Katahdin—a being described as “a part of everything in God's creation.” Verne begins to sense an ominous undercurrent as he observes disturbing phenomena, like plant roots unnaturally surfacing and an unusual bullfrog-sized tadpole caught in a net. These unsettling observations propel the idyllic landscape into a more sinister light.
While on a tour of the paper mill, the couple seeks evidentiary support but finds that the hazardous chemicals used in the process are purportedly contained within the mill. Isely reassures them of the water’s purity, but Verne, ever vigilant, notices mercury on Maggie's boots—an insidious mutagen linked to severe birth defects and nerve damage. Understanding the urgency of the situation, he resolves to take blood samples from the Opies.
The tension spikes as the Nelson family is brutally attacked at their campsite by Katahdin—the monstrous bear, marked by its grotesquely mutated skin. In a frantic attempt to escape, young Paul meets a tragic end at the paws of the beast. Meanwhile, Verne struggles to collect blood samples when Isely and Sheriff Bartholomew Pilgrim arrive with misconceived intentions to arrest Hawks and his men, believing them responsible for the tragedy. Disastrously, Hawks manages to evade capture.
In an effort to uncover the chilling truth behind the attacks, Verne, Maggie, and Ramona commandeer a helicopter, seeking answers at the Nelson camp. While Verne and Ramona find telltale scratch marks on trees, Maggie stumbles upon two bear cubs ensnared in a salmon poacher’s net—one still alive. Determined to prove the environmental contamination, Verne is met with natural forces as the weather turns hostile. The pilot, Huntoon, ultimately refuses to navigate in such perilous conditions.
They seek refuge at Hector's home, where Verne establishes makeshift medical care for the cub. Overwhelmed with anxiety, Maggie confides in Verne about her pregnancy and reveals her consumption of contaminated fish. Tension escalates as Isely and Sheriff Pilgrim arrive upon seeing the mutant cub—Isely finally acknowledges that Hawks and his people are not to blame for the preceding attacks. But before they can process this revelation, the vengeful Katahdin descends on their encampment, wreaking havoc and driving everyone into subterranean tunnels for survival.
After an eerie silence, Sheriff Pilgrim pokes his head out—only to meet a grisly fate at the jaws of Katahdin. With their numbers dwindling, the remaining survivors strategize an escape. Unfortunately, their endeavors through the woods meet gruesome challenges; Huntoon also suffers a dire fate, leading the group to hastily traverse to an impending river.
With desperation mounting, Verne finds a Winchester Model 1892 rifle in a cabin. As he processes the gravity of the situation, notable confrontations ensue, culminating in a harrowing struggle against the horrific creature. Amidst the chaos, Verne channels his knowledge and bravery, relentlessly attacking Katahdin with arrows and determined stabs until the creature collapses beneath the lake’s surface.
The day following these harrowing events, Verne and Maggie soar over the landscape in a helicopter, blissfully unaware that another mutated animal lurks in the shadows of the forest, anticipating its next prey. The entwined narratives of survival and environmental strife reflect a larger commentary on the delicate balance between nature and humanity, echoing through the chilling aftermath.
