Stargate: Atlantis Episode 5.18 Identity

When it's observed that Dr. Jennifer Keller is behaving very strangely, Dr. Carson Beckett is asked to examine her. But extensive testing reveals nothing abnormal and they fear she may be suffering from a mysterious psychological disorder. Unbeknownst to them, Keller has traded bodies with that of a notorious criminal, Neela Casol, and finds herself at the mercy of a village magistrate, on a distant planet, about to be executed for her crimes.

McKay's investigation leads him to suspect that Dr. Keller and Neela have switched consciousnesses by means of an Ancient communications tool. In a race against time, Sheppard and his team must locate Keller, along with the alien device. But they'll need Neela's help to do it.