A patient disappears at night at a hospital without leaving a trace. An investigation is started by the Director of the hospital after the body of a patient turns up dead. DNA evidence is used to find the murder.

Niki receives a letter from a woman, Elvira Strasser, who says she was convicted for a crime she didn't commit. Not long afterwards, Niki discovers that the woman has apparently committed suicide even though she only has six months left of her sentence. Against Marc and Kunz's wishes, Niki goes undercover in the prison to find the truth, but finds herself in danger.