Ronan Carver, [Mark Hapka], and Beatrix Carver, [Jessica Rothe], each receive a message from their father, Alex, [Yorgo Constantine], asking them to meet him at his house. Curious, they converge there separately to find him missing and a strange, ball-shaped device left behind. When Beatrix's friend Harry arrives, he joins the search, even as Ronan distrusts him. They decide to contact the police, but Captain Stone, [Davi Jay], offers little help since there’s no evidence of wrongdoing, and he directs them to the building Alex described, which appears abandoned.
Inside, they discover the building is empty and covered in graffiti about alternate timelines and histories. At first dismissive, they soon realize the graffiti describes a real phenomenon: the building has transported them to an alternate Earth devastated by nuclear blasts. A roving patrol shoots pedestrians, and the trio barely escapes back inside, where they meet Polly, [Constance Wu], a traveler between worlds. Polly explains the rule: the building transports anyone inside to a new, random world if they return to it in exactly 36 hours. In hostile worlds, she advises staying inside the building and not interacting with locals. The group sleeps to pass the time.
They’re woken by an armed patrol and taken to the leader of the local settlement. Polly takes charge and convinces him that they are merely confused travelers. The leader is Captain Stone, [Davi Jay], whose distrust of outsiders who know his name and past occupation grows dangerous, and he orders Tinkler, [Michael Monks], a local surgeon and gadget-maker, to interrogate them.
Tinkler shows them footage of a terrorist attack via a suitcase nuke, and Ronan and Beatrix are horrified to recognize their father as the perpetrator. Ronan breaks free, knocks Tinkler unconscious, and frees the others. Desperate for revenge, Tinkler sets off with his own suitcase nuke to destroy the building.
The group returns to the building just in time to escape the blast and are transported to the next world. This world is technologically advanced, and the group splits up to learn more about it and about Alex. Ronan and Polly explore Alex's house in this world, which turns out to be empty. Ronan reveals that he blames himself for his mother's death, and Polly seems confused when he jokingly quotes slang from her world back to her. Ronan shows the device, and she says that it could only be from the core world where the building was constructed. They flee when they find a hidden warning, and a team of assassins attack them.
Beatrix and Harry discover that in this world Alex was killed and Ronan apparently never existed. Harry sends evidence of his double's wrongdoing to the media, and they return to the building, where Tinker has apparently taken Ronan and Polly hostage. Beatrix threatens to kill Tinker, but Ronan convinces her to allow Tinker to use an invention that hacks into the building. Impressed with the device, Alex reveals himself after stepping out from a secure elevator.
Alex tells them that he and their mother were not born on their home world. Tinker returns from the elevator, yelling not to trust Alex, and Alex shoots and kills Tinker. Alex leaves the building after telling Ronan and Beatrix that they must find their mother, who they insist is dead, in order to return to the core world. After the building transports them again, three different versions of Polly convene in secret and two of them play rock-paper-scissors to determine who will accompany the rest of the group into the next world.
