Twelve-year-old Chess Greystone and his younger siblings, Emma and Finn, live with their mom in Ohio. Their dad died eight years earlier, and only Chess remembers him. The kids return from school one day to find Mom deeply troubled. She’s watching a report about three kidnapped siblings in Arizona.
The Greystone children are shocked to learn the missing kids, whose last name is Gustano, have the same first and middle names as each of them, as well as the same dates of birth. Chess overhears Mom having a strange conversation at three in the morning with someone named Joe. The next day, she tells the kids she’s leaving on a business trip and may be too busy to call them. They will stay with one of Mom’s PTA acquaintances, Ms. Morales, and her teen daughter, Natalie.
The kids and Natalie make trips to the Greystone house to feed the cat every few days. They discover Mom’s phone and computers are still there, and they find a code on Mom’s laptop. They also find secret rooms and passageways beyond Mom’s basement office that take them to a world they don’t recognize. Ms. Morales is a private investigator who often spies on cheating husbands, so Natalie is sure there is some sort of relationship problem at the core of Mom’s disappearance. The Greystone siblings assure her she’s wrong.
The kids return to Natalie’s house with Mom’s laptop. When they finally crack part of Mom’s code, they learn the world they’ve visited—and the world where Mom is currently imprisoned—is an alternate reality. Mom and Dad had lived there and tried to fight the oppressive leadership. When Dad was killed for this, Mom took the kids and fled to their current reality. She knew that they would have doppelgängers there, so she did her best to ensure their paths would never cross.
But when she heard the news report about the kidnappings in Arizona, she knew enemies from her old reality were after her. They had inadvertently kidnapped her doppelgänger’s children. So Mom returned to the alternate reality to try to make things right for the Gustanos.
The Greystone kids and Natalie return to the alternate reality, too, where they discover Mom is on trial. The corrupt leader, who is actually Ms. Morales’s doppelgänger, is using false recordings to make the townspeople think Mom is confessing to crimes. The kids meet Joe, the man from Mom’s phone conversation, and he helps them get to her. She tells them the most important thing now is for them to save the Gustano kids, who are in the same building. Mom says she’ll meet them back at home. The kids reluctantly leave Mom and help the Gustano children escape back into their normal reality.
In the process, the real Ms. Morales gets trapped in the alternate reality. The kids hope by solving the rest of Mom’s code, they can save their mother and Natalie’s mom.