Five years have passed since Tommy Jarvis defeated the infamous Jason Voorhees in Part 4, but the psychological scars from that encounter remain profoundly etched in his mind. He is plagued by relentless nightmares of the notorious murderer, leading him to a series of stays in psychiatric hospitals where treatment has yielded minimal results. Eventually, Tommy finds refuge at the Pinehurst Halfway House, under the care of Dr. Matt Letter and his dedicated assistant, Pam Roberts, who are both determined to help him confront his inner demons.
At the halfway house, a diverse group of troubled teens resides, including Eddie, a boy deeply in love with his girlfriend Tina, who shares a similar passion; Jake, who struggles with a stutter; the rebellious and sharp-tongued Violet; introspective Robin; the voracious Joey; and youthful Reggie, whose grandfather George serves as the cook at the facility. Their easygoing lives are disrupted by their grumpy neighbor Ethel, who takes offense at Eddie and Tina's romantic escapades on her property.
To maintain harmony, Dr. Letter enforces rules that prevent the teens from leaving the premises. However, their boundaries are put to the test when Vick, a disturbed patient, becomes increasingly agitated due to Joey's incessant teasing. In a tragic turn of events, Vick succumbs to his rage and gruesomely murders Joey with an axe, which leads to his subsequent arrest.
As time goes on, an ominous atmosphere envelops Pinehurst. In a chilling discovery, two greasers, Vinnie and Pete, are found dead in their car after it breaks down on the roadside. The following night, a diner waitress and her boyfriend meet a cruel fate at the hands of an unseen assailant. The local sheriff theorizes that Jason Voorhees has returned from beyond the grave to exact vengeance, and the growing suspicion casts a dark shadow over Tommy, placing him as a potential suspect in the unfolding chaos.
Ignoring Dr. Letter's guidance, Eddie and Tina venture into the woods for a night of indulgence, completely unaware that they are being spied upon by Raymond, Ethel's farmhand, who soon meets a dire end. The next morning, Eddie is devastated to find Tina brutally murdered, her eyes pierced by garden shears. His horror escalates as he himself is confronted by the mysterious killer, leading to his gruesome demise as well.
Meanwhile, Reggie convinces his grandfather George to allow him to visit his older brother, Demon, who has just returned to the area. Pam decides to accompany them, bringing Tommy along for support. Their outing is marred when Tommy has a clash with Ethel's son Junior, prompting Tommy to hide in the woods. As Pam and Reggie search for him, Demon's day takes a catastrophic turn when he is struck by a sudden bout of illness, forcing him to seek solace in an outhouse where both he and his girlfriend, Anita, meet a horrific end.
The string of murders escalates, casting Pinehurst into disarray and panic. Amidst the carnage, as Tommy navigates this treacherous environment, it becomes evident that Jason may not be the only wicked force driving the bloodshed. Upon returning to the facility, Pam and Reggie are greeted with an unsettling revelation: both Matt and Reggie’s grandfather have mysteriously disappeared. Pam takes the initiative to investigate, leaving Reggie in the care of Violet, Jake, and Robin. Meanwhile, further slaughter unfolds in Tommy's quarters, as Ethel and Junior meet an unfortunate fate, soon followed by Jake, Robin, and Violet, all falling victim to an unforeseen tragedy while Reggie was momentarily asleep.
When Pam returns, she finds Reggie awakened to a ghastly sight: the remains of his friends scattered around Tommy’s room. Just then, the door swings open, revealing what seems to be Jason himself or a terrifying doppelgänger. Pam and Reggie race through the woods, confronting various horrors as they stumble upon the gruesome remains of Matt and Reggie’s grandfather, a stark reminder of the lurking danger. Their frantic pursuit leads them to a barn where a showdown ensues with their shadowy antagonist.
In a desperate attempt to turn the tide, Pam and Reggie set a trap, only for the “Jason” figure to seize Reggie and hold him tight. In the midst of the chaos, Tommy awakens, grabbing Roy’s machete and hurling it back at the assailant - severing his hand in the process. As "Jason" crashes through the loft window, his gruesome fall onto a tractor harrow marks his swift end.
However, a deeper mystery unfolds as beneath the iconic hockey mask lies the deranged face of Roy Burns, a paramedic who was traumatized by his son’s earlier murder. Consumed by grief and vengeance, Roy had been enacting revenge in a style reminiscent of Jason Voorhees.
In a future hospital visit, Tommy once more battles with a haunting hallucination of Jason. Determined not to be overwhelmed by fear, he casts the image aside. When Pam makes her way into the room, Tommy creates a diversion by smashing a window to simulate an escape. As Pam rushes in, he reappears, this time clad in Roy’s hockey mask and wielding a kitchen knife— a haunting reminder that madness can lie within even the most unsuspecting individuals.
