The story begins in Kearney, Nebraska. 27-year-old Mark Schluter nearly dies after his truck skids out and flips over. Mark is pulled from the wreckage and rushed to a nearby hospital. His older sister, Karin, receives word of the accident and leaves her job to care for her brother. They are very close, as Karin essentially raised Mark after her intensely religious mother and neglectful father did not. Karin arrives at the hospital and learns that her brother is in a temporarily induced coma after suffering brain damage.
A few days later, Mark wakes up. Initially everyone is thrilled, but then he begins saying strange things about Karin. He says that she is an imposter. Devastated, Karin asks the doctors what is going on. They tell her he is suffering from Capgras syndrome, a condition common among patients with brain injuries. The syndrome makes him believe everyone around him is an imposter of the person they claim to be.
Karin is deeply concerned for her brother's well-being. Mark is also in possession of a strange, unsigned note that reads: "I am No One / but Tonight on North Line Road / GOD led me to you / so You could Live / and bring back someone else." Mark also has a dim memory of seeing a mysterious white figure in the road that he swerved away from.
Mark views other close relationships in his life with the same sense of scrutiny, including the one with his dog Blackie. However, he is able to see that his friends, Duane and Tommy, and girlfriend, Bonnie, are who they say they are, which suggests that those relationships are less meaningful. Mark's physical condition gradually improves in the hospital and he is transferred to a rehabilitation center. There, he is cared for by a mysterious nurse's aide named Barbara Gillespie. Mark and Barbara develop a close bond. After a few weeks of rehab, Mark is allowed to return home. Barbara makes regular visits to see how Mark is doing.
Karin also reconnects with her ex-boyfriend Daniel Riegel, who was a childhood friend of Mark's. Daniel is a passionate environmentalist who is fighting against a development company that wants to build on a local Sandhill crane habitat. Daniel comforts Karin and they begin to rekindle their past relationship. Meanwhile, Mark's case attracts the interest of neurologist Gerald Weber. Weber travels out to Kearney to observe Mark and use his case as the basis of a new book. He struggles to connect to Mark, who regards him with a great deal of suspicion.
Mark feels unsafe in his home and becomes convinced that someone was trying to kill him the night of his crash. He becomes increasingly paranoid and reaches out to a crime show hotline to find the author of the note. Karin is disturbed by Mark's behavior and asks Weber to come back for another visit with Daniel. In this time, Weber also falls for Barbara, despite being warned by Karin that she may not be who she appears to be. Weber proves to be unhelpful in curing Mark. Karin also reaches out to Robert Karsh, another ex-boyfriend who she cheated on Daniel with in the past. Karsh is an untrustworthy real estate developer who is involved in the project that Daniel is working against. Mark becomes increasingly isolated and makes an attempt on his life by overdosing on pills. He narrowly survives this incident.
Karin continues to see both Karsh and Daniel. She spends time with Karsh but they never kiss or sleep together. Instead, they talk about his job and her brother. Her relationship with Daniel deteriorates as they hit various familiar roadblocks. This culminates in a big disagreement after they go out to dinner at a Chinese restaurant. Karin betrays Karsh by giving Daniel key information about the development company's plans for the refuge, dealing them a major blow in the court case being pursued against them by the crane preservationists. Karsh, humiliated, lashes out at her after the case, saying this is only a minor setback. Daniel breaks things off with her and leaves Kearney, saying he was aware she was seeing Karsh.
Weber questions Barbara about who she actually is. She reveals that she is a journalist who was assigned to write about the Sandhill crane migration pattern. She also tells Weber that she was the figure in the road that night. Barbara tells Weber everything that happened that night. She says that she was on that road that night because she was trying to commit suicide. Mark swerving out of the way ultimately saved her life. Feeling an intense sense of obligation, she spent the year watching over him as his body and mind recovered. Barbara adds that Mark wrote the note to himself just before falling into the coma.
At the conclusion of the novel, Karin and Mark, who now recognizes her as his sister, happily reconcile. Mark forgives Barbara and attempts to resume his semblance of a normal life. Mark has a final conversation with Weber about the accident. Mark says he is happy to have been Barbara's guardian. He says that the crane refuge will need researchers. He asks Weber if Barbara would consider it and if he might be able to reach out to her again. Weber gently encourages him to try. The book ends with Weber flying home and searching through a crowd of people for Barbara, hoping she is at the airport, holding a sign with his name on it.
