| Director | Michael Shanks |
|---|---|
| Budget | $17 million |
| Box office | $34.7 million |
A man returns home from searching for a missing couple with his two dogs. A short time later he is awakened by the dogs barking and when he arrives at their kennel, the two dogs are painfully fusing together.
A short time later, Millie Wilson gets a job teaching elementary school English and moves to the town with her longtime boyfriend, aspiring musician Tim Brassington, who has been emotionally distant since the recent passing of his parents. Shortly before the move, Millie proposes to Tim at a going away party held by their friends but he hesitates to respond. The next morning Millie meets her new coworker, Jamie, and he tells them about the walking trails near their home. While hiking near their home, they fall into an underground cave during a rainstorm and decide to camp inside for the night. Tim drinks from a pool of water but Millie refuses. Upon waking up the next morning, Tim and Millie find their legs partially stuck together by a type of gooey material but dismiss the incident after separating, believing it to have been caused by mildew.
Tim experiences episodes where he is inexplicably physically drawn to Millie, confusing and frustrating them both. A doctor dismisses the incidents as panic attacks and prescribes him muscle relaxant pills before mentioning that a local couple recently went missing. Jamie shows up at their home to welcome them to the neighborhood, and she invites him in for dinner. The couple recounts their experience at the cave, which Jamie explains was a New Age church before it caved in. Millie drops Tim off at the train station for a gig, but Tim experiences another episode and goes to Millie's workplace. She angrily confronts him in a bathroom stall, where they end up having sex, after which their genitals become stuck together. After they painfully force themselves to separate, Jamie spots them.
Millie later visits Jamie's house to apologize and Jamie offers her some water, which she drinks. She opens up about some of the problems she and Tim have been facing in their relationship. Jamie relates the theory of the nature and origin of eros in Symposium and encourages her not to let go of her "other half", then reminisces on his relationship with his seemingly deceased husband. Millie leaves after spotting a disoriented Tim outside. Using exif data from their social media photos, Tim discovers that the couple visited the same cave he and Millie fell into.
That night, Tim and Millie are supernaturally drawn together until their arms become fused. They consume Tim's pills which reverses the fusing before passing out. Tim awakens tied to a chair, where Millie saws through their fused limb. Millie decides to drive them to the hospital, but realizes she has left her keys at Jamie's house. Tim returns to the cave and finds the missing couple fused together as a grotesque hybrid. Millie enters Jamie's house and finds a television playing a wedding tape and learns Jamie is the two married men fused together as part of the church. Jamie promises Millie they are happier after "becoming whole" and implores Millie to complete her fusion with Tim. She flees back to her and Tim's home after Jamie cuts her.
Millie reunites with Tim in their driveway. He mentions the fused couple he saw in the cave, deducing that it was a result of both parties resisting the fusion, and he prepares to slit his throat to save Millie. Her wound from Jamie nearly causes her to bleed out but Tim fuses his arm to her wound. Accepting their fate, the two affirm their mutual love and embrace as they fuse into a singular being. That weekend, Millie's parents arrive for a visit, and they are greeted by the now fused Millie and Tim as one androgynous person.