Parallels

Ronan and Beatrix each receive a message from their father, Alex, asking them to meet him. They converge at his house and discover he is missing, and in the process Ronan finds a strange, ball-shaped device.

Captain Stone offers little help due to the lack of evidence and directs them to the abandoned building Alex described. That building becomes their lead as they begin a directed search for his whereabouts.

They arrive at the building and find it empty and covered in graffiti describing alternate timelines and histories. At first they dismiss the graffiti as nonsense, but they soon realize the material hints at something extraordinary about the building.

The building suddenly transports them to an alternate Earth devastated by nuclear blasts. Outside, a roving patrol shoots pedestrians, forcing them to flee back inside, where they meet Polly, a traveler between worlds who explains how the building works.

Polly explains that the building moves them to a new world if they return to it within 36 hours. She cautions that in hostile worlds they should stay indoors and avoid contact with locals. The group sleeps to pass the time.

They are awakened by an armed patrol and brought to meet the settlement's leader, Captain Stone. Polly defuses the situation and convinces him they are merely confused travelers, but Stone grows distrustful of outsiders and has them detained while Tinker interrogates them.

Tinker 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 Tinker unconscious, and frees the others. Desperate for revenge, Tinker sets off with his own suitcase nuke to destroy the building, and the group returns to the building just in time to escape the blast and are transported to the next world, though they do not yet realize that Tinker accompanies them.

They arrive in a technologically advanced world and the group splits to learn more. Ronan and Polly explore Alex's house in this world and find it empty. Ronan reveals he blames himself for his mother's death, and Polly seems puzzled by some of the slang he quotes, while the ball device hints at its core-world origin.

Beatrix and Harry realize that, in this world, Alex was killed in an accident and Ronan never existed. Harry hacks into the world's systems and sends evidence of his double's wrongdoing to the media. They return to the building and find that Tinker has apparently taken Ronan and Polly hostage.

Beatrix threatens Tinker, but Ronan convinces her to let Tinker use an invention that hacks into the building. Impressed by the device, Alex reveals himself after stepping out from a secure elevator.

Alex explains that he and their mother were not born on the world they think of as home. He tells the children that they must locate their mother to return to the core world.

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 into the next world.