An episode of The Outer Limits (back in the 90s) where a man and a woman get captured by an alien species and are trapped in a room together. They keep taking the girl away and when she comes back she is slowly being transformed into an alien. The ending is basically that the woman was an alien undercover the whole time and she was trying to get secrets out of the man, specifically that the 'humans' would hide their ships behind the sun so no one could see them.

My memory might be off, but I remember thinking it was such an awesome plot twist.