During a work expedition in Nepal, photo-journalists Ted Harrison and his girlfriend Marjorie encounter a terrifying creature when they are attacked by a werewolf. The beast snatches Marjorie away, and despite Ted’s frantic efforts to save her, he gets bitten in the shoulder during the struggle. Fortunately, Ted manages to kill the werewolf with his shotgun, but tragically, Marjorie does not survive the attack. This horrifying event leaves Ted deeply traumatized and seeking solitude, leading him to move into a remote trailer nestled deep within the woods.
As time passes, Ted’s life takes a darker turn. He eventually invites his sister Janet and her young son Brett for a meal at his secluded lakeside home. When Janet arrives, she notices something strange — the family dog, Thor, immediately runs into the woods, clearly disturbed. Thor’s behavior prompts him to pick up a scent, which leads him to uncover a grisly sight: human remains hanging from a tree branch, a gruesome reminder of the mysterious deaths plaguing the area. Meanwhile, Ted, in an effort to hide the truth, deceives Janet by telling her that Marjorie left him and went back to Seattle. That evening, Janet invites Ted to stay with her and her family, but he declines and insists they leave before sunset, sensing that something dangerous lurks in the dark.
The following day, local authorities discover the remains of several missing hikers and a Forest Ranger near Ted’s trailer. The authorities are concerned about the growing number of disappearances; fear and suspicion start to mount in the community. Ted, feeling the weight of guilt and fear of being accused, calls Janet, and she arrives just in time to see Thor acting suspiciously toward her brother. Thor’s hostile attitude raises alarm, especially when later that night, he follows Ted into the woods and uncovers a shocking sight: Ted has turned into a werewolf and is chained to a tree. Thor’s instinctive hostility and warning signs serve as crucial clues to Janet that her brother’s strange behavior is linked to the sinister creature in the woods. Later, Janet ventures into the forest searching for Thor, but Thor, understanding the danger, guides her safely back to the house.
The next day, Janet is overwhelmed when she sees a TV news report exposing more victims and personal tragedies in the area. She confronts Ted, demanding the truth about his silence and his strange behaviors. Ted tries to warn her, hinting that the murders are being committed by a wolf — a warning she dismisses. He also carefully suggests she pay attention to Thor’s sudden and unusual changes in behavior, but she refuses to believe him fully. As sunset approaches, Ted prepares himself to chain himself up again, desperate to contain the beast within. Recognizing Ted’s distress, Brett lets Thor out of the house, and the loyal dog runs off into the woods determined to find Ted. Thor soon discovers that Ted was too late to lock himself securely, and the tension escalates.
Thor, sensing the impending danger, barks furiously until Brett lets him out. Thor rushes into the woods, where he finds Ted, who is in the midst of transforming into the werewolf. Ted acts aggressively and tries to attack Thor, resulting in a brutal fight between man, beast, and dog. Janet, alarmed by the commotion, switches on the deck lights and frightens Ted away. She finds Thor injured from the encounter and calls the sheriff. During her investigation, she finds a disturbing book filled with gruesome pictures of Marjorie’s corpse and Ted’s previous victims, alongside a journal where Ted documents his inner turmoil and his desperate hope to find a cure for his "disease." The journal reveals his growing despair and wish for peace near his family.
Later that night, a traveling salesman who previously attempted to frame Thor for an attack returns to Janet’s yard with malicious intent, aiming to kill Thor. However, Ted intervenes and fatally wounds the salesman, further complicating the situation. The sheriff later questions Janet about Thor, and she learns that the salesman was found dead, mutilated in a manner that suggests an attack by a wild animal. Despite her suspicions, the sheriff dismisses the idea of a wolf and advises her to send Thor to the pound. Janet, however, remains unconvinced, especially when Ted begins to provoke Thor into attacking him, which results in Thor being taken away.
That night, Janet confronts Ted in the woods. He accuses her of knowing the truth all along and not trusting him. As Ted begins his horrifying werewolf transformation, Janet retreats to the safety of her home and retrieves a revolver hidden in the kitchen. Meanwhile, Brett manages to sneak out, release Thor from the pound, and reunite with him just as Ted prepares to attack Janet. A fierce battle ensues inside and outside the house, with Ted throwing Thor across the room in a desperate move. Brett nearly becomes Ted’s next victim, but Janet fires several shots, forcing Ted to retreat into the woods. Thor, injured but determined, tracks Ted into the forest at dawn, where Ted, now human again, emerges from behind a tree injured and battered. In a final act of justice, Thor, still protective and fierce, kills Ted to stop his murderous rampage.
After the ordeal, Janet feels remorse for having sent Thor to the pound and mourns the chaos they endured. As a moment of relief washes over them, Thor suddenly growls — only to reveal it was just a nightmare, not reality. Janet, Brett, and Thor are left to recover from their terrifying experience, knowing that the threat of the werewolf has finally been ended — but the scars it left behind are a reminder of how quickly lurking evil can emerge from the shadows.
