When gifted homicide detective Ryan Mills is recruited to a top-secret FBI division known as S.P.E.C.T.E.R., the world of the supernatural becomes more real than she could have ever imagined. She is introduced to an incredible device known as Lazarus, which can replay the events of a crime with the smallest pieces of available evidence. With the help of her new partner, Special Agent Sam Masters, and Dr. Matthew Holbrook, the research scientist who created Lazarus, Ryan is plunged into the paranormal exploration of unsolved murders.
But she holds onto a secret that she can't share with her partner or Holbrook. That secret has come back to haunt her when a homicide scene holds clues to someone from her past and has torn at the edges of an 18-year-old cold case: the disappearance of her own sister. The killer scrawls a message meant only for her: yesterday upon the stair, I met a man who wasn't there . . . .
He has come back and only he understands what happened to her sister so long ago. He also knows something about Ryan . . . something even she doesn't know.