At exactly 2:13 a.m., Daniel Carter receives a phone call from his own number. The voice on the other end is his own voice, terrified and desperate, warning him not to open his bedroom door. Seconds later, three knocks echo from the other side.Then a photograph slides underneath the door.It is a picture of Daniel sleeping in his own bed—taken only minutes earlier.Before Daniel can understand what is happening, police surround his apartment building and accuse him of murdering his sister, Emily. There is only one problem: Emily died six years ago.As Daniel searches for answers, he discovers hundreds of photographs documenting his life—some taken years before he supposedly moved into the apartment. Hidden inside the walls are records of forty-six previous versions of himself, each ending in a different mystery.Daniel begins to suspect that his memories are false. But the truth is far worse.The apartment is connected to a secret experiment involving memory, identity and simulated realities. Daniel discovers that the woman he remembers as his sister may not be his sister at all, and that the life he believes he has lived may be nothing more than a carefully constructed illusion.Every answer creates another question.Every memory hides another secret.And when Daniel finally discovers who created the experiment, he realizes the most terrifying possibility of all:He may not be the person trapped inside the experiment.With reality collapsing around him, Daniel must uncover the truth before the forty-eighth version of his life begins.But there is one final secret waiting for him—one that will make everything he believed about himself completely meaningless.