Jessie and Gerald embark on a seemingly peaceful getaway to an isolated lake house in Fairhope, Alabama. Their retreat quickly takes a dark turn when Gerald decides to spice up their evening with a bit of experimentation. After taking Viagra, he playfully restrains Jessie, played by Carla Gugino, with handcuffs to the bed, intending to enact a stranger rape fantasy. While Jessie, feeling a mix of surprise and trepidation, initially goes along with the play, the mood shifts dramatically when she demands he uncuff her. Gerald retorts, >"What if I won’t?" His inflexibility leads to a heated argument, during which he accuses her of neglecting their relationship. Tragically, Gerald suffers a fatal heart attack, leaving Jessie trapped and alone in the house.
Amidst the chaos, a stray dog enters and, in a gruesome turn, bites into Gerald's arm, taking a piece of him with it. Jessie soon finds herself haunted by Gerald's distorted apparition, who taunts her about their troubled marriage and his failures. With mounting desperation, she battles her reality, ultimately pulling a hand free from the cuff. However, her confidence is short-lived when she realizes that she is still very much in peril.
As she grapples with the hallucinations of Gerald and an echo of her former self, Jessie recalls the glass of water left tantalizingly out of reach. With ingenuity, she transforms the tag from her nightdress into a makeshift straw, managing to hydrate herself just in time to sustain her endurance.
Jessie's harrowing ordeal continues as she is plagued by the dark figure known as the "Moonlight Man", personified by Carel Struycken, who embodies her deepest fears. A memory resurfaces—a traumatic encounter with her father, Tom, played by Henry Thomas—from her childhood that ties into her current trauma. As the past and present collide, Jessie is forced to confront the painful truth of her experiences.
Finally, using sheer determination, Jessie orchestrates her escape by inflicting a wound on herself that allows her to slip free of her restraints. After an intense fight against her circumstances, she manages to liberate herself and darts for her car, but the haunting Moonlight Man appears again, leading to a crash that might signify her liberation or the end.
Months later, we see Jessie pen a letter to her younger self, reflecting on her hard-fought recovery and the creation of a foundation to support victims of sexual abuse, funded by Gerald’s life insurance. Still, the vestiges of her trauma linger—each night the spectral figure manifests in her dreams. As she learns of a serial killer, tied to her horrific experiences, Jessie finds herself compelled to confront the past in a courtroom, finally declaring, "You’re so much smaller than I remember," as she steps out into the sunlight, triumphant in her reclaiming of identity and strength.