An elderly widow, Judith Albright, is preparing to celebrate her significant 70th birthday surrounded by her family, including her daughter and grandson, Josh. However, the joyous occasion quickly turns somber when Judith suffers a stroke during the festivities. After a medical assessment, it is confirmed that she has been diagnosed with Parkinson's disease. Determined not to burden her loved ones as her condition progresses, Judith makes the difficult decision to reside in Golden Sun Manor, a nursing home that promises a nurturing environment.
Upon arriving at the manor, Judith finds herself attended to by the compassionate nurse, Liesel, who assists her in adjusting to her new surroundings. As she begins to settle in, Judith forms a close friendship with an eccentric trio of spirited seniors: Ruth, Trish, and Roland. This vibrant group invites her to engage in games of bridge, and the once-lonely Judith discovers a newfound camaraderie that rekindles her zest for life.
However, her initial experiences in the manor take an unsettling turn when her roommate, Annette, warns Judith of an ominous presence lurking nearby. Initially dismissive, Judith soon finds her mind filled with terrifying visions of a monster crafted from tree bark that seems to inhabit her room. While the staff and other residents attribute her vivid hallucinations to the onset of dementia, Judith resolutely believes there is something more sinister at play and vows to uncover the truth behind the monstrous specter.
One fateful night, Judith witnesses the horrifying sight of the bark monster attacking Annette, who tragically succumbs to a stroke shortly thereafter. In her last moments, Annette presents Judith with an ominous message: that they have both been marked, handing her a note scrawled with names. Annette ominously predicts that the bark monster will claim another patient, named Imogen, and ultimately come for Judith herself. Alarmed by these revelations, Judith searches frantically for answers and discovers a voodoo artifact stashed beneath her bed. With no time to lose, she decides to escape the manor.
In a twist of fate, Judith crosses paths with a group of teenagers who bear an uncanny resemblance to Ruth, Trish, and Roland, leaving her perplexed about the reasons behind this bizarre similarity. Attempting to seek solace from her daughter and Josh, she is met with resistance, as her daughter instead insists on returning Judith to the manor. It is during a trip to the diner that Judith encounters a wall photograph featuring Ruth, Trish, and Roland, all of whom look disconcertingly youthful and unchanged from decades prior.
Judith's attempts to convey her concerns about the dangers lurking in the manor fall on deaf ears, as her family believes she is losing her grip on reality. Resolute to protect herself and uncover the truth, she sneaks into the director's office and uncovers startling connections: Roland is related to the manor's director, Ms. Benson, while Ruth also seems to have ties to a male nurse named Gary. These revelations fuel Judith's investigation further as she shares her findings with Josh, who discovers a Celtic sigil etched on Trish's arm—an oak tree symbolizing longevity and regeneration.
The deeper Judith delves into the mystery of Golden Sun Manor, the more disturbing the parallels become. She soon begins to suspect that Roland, Trish, and Ruth are engaged in an evil practice of black voodoo, using innocent lives to maintain their unearthly vitality. In a desperate act of subversion, she switches out her hair strands with Roland's, planning to follow him into the woods to unearth more secrets.
What Judith and Josh stumble upon in the woods is straight out of a nightmare: a spiritual ceremony led by the trio that involves invoking the power of the oak tree and calling for sacrifices. Roland, mistaking Judith's hair for his target, completes the ceremony, only to unleash a monstrous entity from the tree that ultimately claims his life.
Determined to escape the dark fate that awaits her, Judith seizes a bottle of alcohol and threatens Trish and Ruth, demanding they burn the tree down. In a chilling turn of events, Trish and Ruth entice Judith with promises of eternal youth and the chance to escape the inevitable decay of age. In a moment of weakness, swayed by emotions and the longing to hold onto her grandson, Judith ultimately submits to their dark offer, becoming intertwined with their malevolent magic.
In a powerful conclusion, Judith celebrates her 70th birthday once more, her age frozen in time as she embraces her new identity within the sect. Bonded with Josh, who seeks to hold onto his grandmother through any means necessary, they both yield to the dark enchantments of the manor, now marked by the same Celtic sigil that once bore significance in their journey. The film’s haunting final moments reveal Judith, youthful and revitalized, dancing gracefully beneath the backdrop of the colossal oak, illustrating not only her transformation but the tragic consequence of valuing eternal life above the sanctity of innocence.
