A dying alien organism attaches itself to B'Elanna's nervous system, and the Doctor doesn't know how to treat her. He enlists the help of a holographic recreation of the Cardassian Dr. Crell Mossett, an exobiology specialist and war criminal, to save her life. B'Elanna refuses all treatment when the Bajoran crew informs her that Crell was responsible for sadistic war crimes during the occupation. The Doctor experiences a moral quandary about using and retaining Crell's program.