In 1986, young Adelaide Thomas wanders away from her parents at the Santa Cruz Beach Boardwalk and enters a funhouse. She encounters Red, a doppelgänger of herself, in the house of mirrors. Following the encounter, she suddenly stops speaking, and withdraws from her family.
Thirty-three years later, Adelaide, now an adult, reluctantly joins her husband, Gabe Wilson, and their children, Zora and Jason, on a vacation. They are joined by friends, Josh and Kitty Tyler, along with their twin daughters, Becca and Lindsey, at the beach. During their outing, they observe paramedics removing the body of an elderly man holding a sign that matches one Adelaide had seen on the day she encountered her doppelgänger, as a child. Later, Jason spots an individual bearing a striking resemblance to the deceased, motionless with outstretched arms and bloodied hands.
That night, Adelaide relays her story to Gabe, when the lights suddenly go out. Jason notices a family of four in the driveway, who break into the house and corner the Wilsons. The intruders are the Wilsons' doppelgängers: Red (Adelaide's doppelganger), Pluto (Jason's pyromaniac, facially scarred double), Umbrae (Zora's sadistic double) and Abraham (Gabe's animalistic double). Red is the only double who can speak, albeit in a raspy voice. She explains that they are called the Tethered, as they share a soul with their counterparts and have come to untether themselves. The Wilsons are separated and terrorized by their doppelgängers. Jason discovers that Pluto mirrors his actions. After Gabe kills Abraham, the family escapes.
Meanwhile, the Tyler family is attacked and murdered in their home by their doppelgängers. The Wilsons arrive and are attacked as well, but they manage to overpower and kill the Tylers' doubles. They watch the news and learn that the Tethered have been murdering their equivalents across the city then joining hands to form a human chain (similar to the one from the Hands Across America demonstration) that surrounds the city. The Wilsons decide to drive along the coast and escape to Mexico. While they are leaving, Umbrae intercepts the car but Zora throws her off the car and kills her Adelaide goes separately to comfort Umbrae as she dies.
Arriving at the boardwalk, the Wilsons find many townspeople slaughtered. The road is blocked by a burning car. Jason, realizing it is a trap set by Pluto, orders everyone out of the car. Before Pluto can ignite the family's car, Jason walks backwards, causing Pluto to do the same and thus walk directly into the fire, to Adelaide's sadness. While the Wilsons are distracted by Pluto burning to death, Red appears and snatches Jason away. Adelaide chases Red to the funhouse where they first met and finds a secret entrance that leads to an underground facility overrun by white rabbits, where she finds Red in a classroom.
Spoken languages: English,French,Hungarian,Polish,Italian
Subtitles: Danish,Swedish,Norwegian,Finnish,Dutch,Greek,Hungarian,Polish,English,Romanian,Cantonese
