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.