TVTropes on Maybe Magic, Maybe Mundane and Eerie Indiana

[froodle]

The Eerie, Indiana episode "Heart on a Chain" is the only one in the series that never answers its mystery. A shy, terminally ill girl has a crush on a devil-may-care boy, who dies in a freak accident. A heart transplant from his fresh corpse saves her. She then begins acting increasingly bizarre (with a lot less self-control). It is left completely unclear until the end whether his heart's personality has taken over hers (as the Agent Mulder believes), or whether guilt has made her not herself.