Millie Wilson, Alison Brie, takes a new teaching job in a quiet countryside town and moves there with her longtime partner, Tim Brassington, Dave Franco, an aspiring musician who has grown emotionally distant after the recent death of his parents. Their life together begins with a mix of hope and hesitation: attendance at a going-away party ends with Tim faltering when Millie asks for a commitment, leaving her feeling both hopeful and hurt. Soon after, they set out to settle into their new home, but nature quickly becomes more than a backdrop than a mere setting; a heavy rainstorm drives them off the road and into an underground cave, where they decide to spend the night.
In the dim, damp shadows of the cave, the couple faces a cascade of unsettling details. Tim drinks from a pool of water, while Millie refuses, sensing danger in the unknown depths. Tim is haunted by a traumatic memory: the night he discovered his mother in bed with his father’s decaying corpse. By morning, the pair realize their legs have become partially fused, a disturbing sign that they cannot ignore. They manage to separate, but the incident marks the beginning of a strange series of physical and emotional disturbances that will strain their relationship and push them toward a fate neither could have anticipated.
As days pass, Tim finds himself inexplicably drawn toward Millie in moments of distress, a compulsion that confuses and unsettles them both. A doctor is consulted, but the physician attributes Tim’s symptoms to panic attacks and prescribes muscle relaxants, all while dropping a quiet rumor of a local couple, Simon and Keri, who have recently vanished. The couple next door, Jamie McCabe, Damon Herriman, appears during a welcome visit and shares the cave story, recounting that the cave’s cave-in had ties to a New Age church. The encounter leaves Millie uneasy and Tim increasingly unmoored. Later, when Tim drifts away during a gig drop-off at Millie’s workplace, a confrontation in a bathroom stall ends with a startling, intimate moment: they have sex, and afterward, their genitals fuse together. After a painful struggle, they separate, but Jamie witnesses the scene, and the strands of their fate begin to intertwine more tightly.
Millie seeks to understand what’s happening and heads to Jamie’s house to apologize. Jamie offers her water and, in a quiet and unsettling exchange, speaks of Aristophanes’ theory of eros and the way lovers can be “made whole.” He hints at a past relationship with a husband who seems to have vanished, a memory that hints at a larger pattern at work. Millie leaves abruptly when she spots a disoriented Tim outside, unsettled by the sense that something supernatural is at play. Tim, meanwhile, uncovers a troubling thread: using the metadata from their social media photos, he discovers that Simon and Keri visited the same cave after noticing the same symbols that surrounded the cave on their own explorations. He tries to convince Millie that the fate that befell Simon and Keri could befall them as well, but his warnings fall on skeptical ears.
That night, the bond between Millie and Tim deepens in a perilous way: a second, more dangerous fusion occurs as their bodies contort and their arms fuse. In a panic, they take Tim’s pills to blunt the pull of the fusion and fall unconscious. Tim wakes to find himself bound to a chair, while Millie, driven by fear and a fierce will to survive, uses a tool to sever their fused limb. She intends to drive to the hospital, but a missing set of keys at Jamie’s house complicates the plan. Tim slips back into the cave and discovers a grotesque scene: Simon and Keri are partially fused into a grotesque humanoid, with Simon’s half appearing as if it were dying. In Jamie’s house, Millie confronts a disturbing tableau—a wedding tape featuring two men she does not recognize and a ritual in which the two halves finally fuse. Jamie, revealed as the product of a failed fusion, promises that “becoming whole” makes them happier, and he pleads with Millie to complete the joining of their lives with Tim. When she resists, he injures her arm in the same way he and others were cut in the ritual.
Millie and Tim reunite in the driveway, and Tim discerns that the movement toward fusion is the result of their resistance to the process. He vows not to end up like the brutalized pair they observed in Jamie’s world, even as the danger intensifies. In a desperate moment, he contemplates taking his own life to protect Millie, a choice that would end their struggle. Millie’s wound, nearly fatal, becomes the catalyst for a dramatic last act: Tim fuses his arm to Millie’s wound, and the two finally accept a shared fate. They begin to move in unison, a slow, almost ceremonial dance toward a singular form. In the final image of the weekend, Millie’s parents arrive for a visit and are met not by two distinct individuals but by a fused, androgynous presence—the living memory of Millie and Tim joined as one.
In this film, the countryside and the cave serve as more than backdrops; they become catalysts for a chilling exploration of connection, loss, and the fear—and allure—of becoming “one.” The story uses the couple’s initial tenderness, their shared traumas, and the unsettling arc of their physical union to probe how love can endure even as bodies break down and fuse into something unrecognizable. It weaves together memory, myth, and a creeping dread that there might be a price to pay for insisting on belonging to another person, and it poses the stark question of what it means to be whole when the self seems to melt into a shared fate. Through its patient escalation, the film invites viewers to sit with discomfort and to witness a love story that refuses to end in separation, no matter how disquieting the path to unity may be. The ending leaves us with a haunting image: two individuals who were once distinct, now inseparable, dancing a final, eerie waltz toward an unknown but unquestionably singular future.
