Sci-fi movie or TV series where earthlings are captured by alien species [duplicate]

Question

It aired somewhere in the nineties I think? It's quite foggy as it is so long ago, so maybe some details are (way) off.

From what I remember it had no happy ending. There is a war against humanity and an alien race. Some people of Earth were imprisoned on a different planet or maybe in an alien ship, and from time to time people were removed and interrogated. One of the people tries to console somebody else, telling there is a big fleet building up behind the moon to win the war, and then that person reveals 'itself' as an alien, shapeshifted into an earthling. It ends with a cry of despair, knowing he betrayed the human race.

Answer 1

This is the The Outer Limits episode Quality of Mercy, featuring Robert Patrick (Terminator 2's T-1000) as Major John Stokes and Nicole de Boer (DS9's Ezri Dax) as Cadet Bree Tristan, two (apparently) human prisoners of alien invaders.

You noted the general shape of the plot, which has the mostly unseen captors gradually transforming Cadet Tristan into an alien via grafts. At the end, with the transformation nearly complete, Stokes tries to offer hope to Tristan, revealing the Earth's secret plan for victory. In turn, Tristan reveals:

She's actually a spy, and that their captors aren't turning her into an alien ... they're turning her back.

Of note, there's a sequel in the following season, The Light Brigade, which is a pretty good episode, even though it undoes some of the impact of its predecessor.