Saw this perhaps 15 years ago…maybe more. I’d say around 1995-2000. Movie was in English, on TV – color.
Plot : I don’t remember the background story but the human race was in some sort of war against an alien race. A human (fighter pilot?) is captured and is in prison. The aliens try to get info from him (where’s the rest of the fleet hidden or something like that) but he doesn’t bulge.
His cell has some sort of round tube about 12in diameter that contains some sort of corrosive water. He gets food that looks like giant crabs or something like that but doesn’t know how to eat them.
At one point, another human, a woman, is put in the same cell has him. He befriends her. She show him how to eat the crab food. Then the aliens get her and when she comes back, she’s transformed a bit into the aliens. Can’t remember how the transformation went but at one point her whole arm is insect like. Turns out the arm is resistant to the corrosive liquid in the pit and she’s able to do something with that (can’t remember what).
The human guy manages to go through the vents and finds the woman on an operating table getting transformed again. When she comes back, she’s almost entirely transformed into an alien. He tries to confort here and tells him the location of the fleet (the info the aliens were after). Something like “they are all waiting behind X or Y and they’ll be here soon, don’t worry”.
Then it is revealed that the woman wasn’t a human getting transformed into an alien.. she tells him “I’m getting back to my own self”… she was an alien all along.
That’s all I can remember!