Tim and Millie are a longtime couple whose relationship has hit a painful plateau. Seeking a clean break from urban stress and hoping to rekindle their romance, they leave the city and purchase a secluded rural home in the country. During a weekend hike through the dense woods behind their property, a sudden rainstorm forces them to seek shelter inside a subterranean cave system. While exploring the dark caverns, they find a pool of glowing, mineral-rich groundwater and unwittingly drink from it to stay hydrated before finding their way back home. The next morning, both Tim and Millie wake up with severe flu-like fever and inflamed welts along their bodies. While taking a hot shower together to ease their aching muscles, they discover something horrific: where their hips and lower torsos rested against one another, their skin has fused into a single raw, fibrous seam. Thinking it is a bizarre allergic reaction or fungal outbreak, they attempt to pull apart, but the action tears through shared muscle tissue and causes unbearable pain and heavy bleeding. As the hours tick by, the supernatural infection rapidly spreads across their anatomy. Blood vessels, nerves, and connective cartilage link up between them, slowly turning two distinct human beings into one conjoined organism. Desperate to keep the nightmare secret while seeking a medical cure, their panic boils over into violent arguments and botched home surgery attempts. As outside neighbors and an offbeat local named Jamie begin checking in on them, the couple realizes that the cave entity is not just fusing their meat, but slowly merging their memories, heartbeats, and conscious minds.

By the climactic final night, Tim and Millie have almost completely integrated into a singular, asymmetrical entity with two heads sharing one massive torso and entangled limbs. An attempt by Jamie to help them ends in bloody catastrophe when the panicking couple, unable to control their combined physical strength, accidentally causes a fatal injury. With sirens approaching in the distance and their shared circulatory system failing from septic shock, the couple finally ceases fighting against their condition. Looking into each other's eyes, Tim and Millie accept their fate and stop resisting the parasite. In their final moments, their anger dissolves into a quiet, tragic intimacy. They realize that their desperate desire to never lose one another has literally brought them to the ultimate stage of unity. As police flashlights pierce the dark living room windows, the camera pulls back on the quiet couple resting peacefully on the couch, completely and permanently bound together in death.