Star Trek Voyager S4 Ep 20 - Gender Transfomrations

Season 4 Episode 20 Vis à Vis
On board Steth's ship, Paris gets to know its pilot a little better with some friendly conversation concerning Steth's previous flying missions. Paris tells Steth about the time when he took his father's shuttle out for a joyride. At one time during the repairs, when Paris' back is turned, Steth briefly transforms into an alien female of another species. The following day, Paris takes Steth into his garage holodeck program to tell him about an idea he had for fixing the alien ship's propulsion system that he came up with, similar to a 20th century car's carburetor. Sometime during the discussion, when again Paris' back is turned to Steth, he transforms yet again into the alien female humanoid, though his voice is still that of his male form. He reverts back to his male form again before Paris notices anything.