In 1986, young Adelaide Thomas visits the Santa Cruz Beach Boardwalk with her parents. While her father is distracted playing a carnival game, Adelaide wanders onto the beach during a thunderstorm and enters a funhouse hall of mirrors called "Shaman's Vision Quest." Deep inside the mirrors, she comes face to face with a living girl who looks identical to her. The encounter leaves Adelaide traumatized and unable to speak. Her parents take her to therapists, who encourage dance lessons to help her regain her voice. Years later, an adult Adelaide travels to her family's vacation home in Santa Cruz with her husband Gabe Wilson and their children, Zora and Jason. Adelaide feels intense dread about returning to the area, haunted by coincidences like seeing the time 11:11 and noticing a homeless man holding a cardboard sign that reads "Jeremiah 11:11." That afternoon at the beach, Jason wanders away and spots a man standing on the sand with his arms outstretched, dripping blood from his hands. That night, the electricity cuts out. Jason alerts his parents to a family of four standing completely still in their driveway, holding hands in the dark. Gabe grabs a baseball bat and tries to intimidate them, but the strangers do not react. Suddenly, they sprint forward, surround the house, and break in through the doors. The intruders force the Wilsons into the living room. To the family's horror, the intruders are exact physical doubles of them. They wear red boiler suits and single brown leather gloves, and they carry sharp golden shears. The mother, named Red, speaks in a raspy, painful croak. Red tells a dark story about two girls who were connected: one girl lived in the warm sunshine eating good food, while her shadow was trapped underground eating raw, cold rabbit, forced to copy every movement the surface girl made. When Gabe asks who they are, Red chillingly replies, "We are Americans." Red orders her family to split the Wilsons up. Her husband Abraham takes Gabe outside, daughter Umbrae chases Zora down the street, and son Pluto traps Jason in a closet. The Wilsons fight back with desperate fury. Gabe kills Abraham with the motorboat engine on the lake, Zora outruns and escapes Umbrae, and Jason discovers that Pluto mimics his physical movements, using this trick to lock Pluto away. The Wilsons regroup and flee to the home of their wealthy friends, Kitty and Josh Tyler. When they arrive, they discover that the Tylers and their twin daughters have already been brutally butchered with shears by their own Tethered doubles. A bloody battle breaks out inside the Tyler house, but the Wilsons manage to kill all four Tyler doubles. Turning on the television, the Wilsons watch in shock as news reports reveal that millions of Tethered in red jumpsuits have emerged across the country, murdering their surface counterparts and forming an unbroken human chain stretching for thousands of miles ("Hands Across America"). The Wilsons take the Tylers' car and head toward the Santa Cruz boardwalk. Along the way, they are ambushed by Umbrae and Pluto, but Zora kills Umbrae by crashing the car, and Jason tricks Pluto into walking backward into a blazing puddle of gasoline. Suddenly, Red appears, abducts Jason, and drags him into the funhouse where Adelaide first met her double. Adelaide chases Red through the funhouse and discovers a hidden elevator that leads deep beneath the earth into a vast network of abandoned white tiled tunnels filled with thousands of caged rabbits. Adelaide enters an underground classroom, where Red awaits her. Red explains the truth: generations ago, the United States government created the Tethered as genetic clones in underground facilities to control the citizens above like puppets. The project failed because the creators could duplicate the body, but could never duplicate the soul. The government abandoned the clones underground, leaving them to live in misery, eating raw rabbits and mindlessly mimicking the surface world. Red was the only clone who could speak, dance, and think independently, which allowed her to organize the Tethered into a grand revolution to reclaim the surface. Red and Adelaide engage in a brutal, dance-like duel with scissors and a fire poker. Red anticipates every strike with graceful speed, but Adelaide manages to impale Red through the chest, snap her neck, and emit a savage, animalistic roar of triumph. Adelaide rescues Jason from a locker, and they return to the surface, reuniting with Gabe and Zora in a stolen ambulance.

As the ambulance drives away through the mountains, Adelaide looks over at her son Jason and smiles gently. But in that quiet moment, Adelaide has a flashback to the night in 1986 inside the hall of mirrors, and the horrifying truth is finally revealed. In 1986, the underground Tethered girl did not just scare young Adelaide; she attacked her. The Tethered girl choked the real surface Adelaide unconscious, dragged her down into the underground tunnels, handcuffed her to a bunk bed, stole her Michael Jackson "Thriller" t-shirt, and walked up to the surface world to take her place. This means that the Adelaide we have followed, cheered for, and loved throughout the entire movie is actually the Tethered clone who escaped thirty years ago. She learned to talk, learned to dance, lived a normal American life, and had children. Meanwhile, "Red", the croaking villain leading the bloody underground revolution, was the original, real human girl who was stolen from her parents and trapped in the dark for three decades. Red's strange, raspy voice was damaged from crying and choking when she was dragged underground as a child. Jason watches Adelaide carefully. Seeing the strange, secretive look on his mother's face, Jason senses that something is wrong. He slowly pulls his plastic monster mask back over his face, realizing that the monster he feared was his mother all along. The movie closes with a sweeping aerial shot of millions of Tethered holding hands across the American desert, showing that the revolution of the forgotten underclass is complete.