"Nit i dia" narrates the daily life of a young forensic doctor and the conflict between her desire to lead a normal life and daily contact with the harshest reality. The series begins on the day that forensic doctor Sara Grau, married to a senior executive of a multinational, begins assisted fertilization treatment. The same day that, in an autopsy, she believes she recognizes the unidentified corpse of a stranger with whom she had a brief extramarital affair. And also the day that the corpse of a murdered elderly woman appears; a new victim of what appears to be a serial killer. From here, a series of events are triggered that turn the lives of the protagonist and all the characters around her upside down.