One specific SciFi story that really moved me as a child, is a rare episode of a TV series (this might be an adaptation of a written short story) that I watched in the early '90s. Given that I don't know the name of the TV series, it may have been recorded in the '80s or earlier.
The whole story takes part in a prison (a prison cell) in an unknown galaxy. A male human (an officer) is thrown into this cell by unknown alien life forms. As the human tries to survive, the time passes and another human is thrown into the cell, this time a female civilian. She has been captured in a convoy trying to escape from Earth (AFAIR). A bond develops between the two and also does the romance and trust.
The female human is taken for interrogation - upon her return, she shows signs of inhuman features, the aliens are trying to modify her and exchange her human parts for the parts of their species. She tells the male human that they are trying to modify her body in the attempt to assimilate her into their species and later try to pass her as a spy. Apparently, they are making an attempt to assimilate all the man kind if the experiment succeeds.
Time passes, and she begins to look like one of the aliens (I remember that they were a bit taller that the humans, also greenish, a bit bug-like). Something takes over the two and they begin to panic, they realize that they will never get out of the prison.
One specific time the female human begins to panic and speaks of a dying human race remaining on the Earth, and the aliens taking over the Earth by conquering the mankind. At this point the male human confides to the female human that there is a large number of troopers waiting to attack the alien colony, they are hiding behind a moon of some planet in the milky way. They are only waiting for his command or the attack on Earth to strike down the aliens.
The female restores her faith, but is quickly take away again for a final change. After her return the change is complete, and she looks completely like one of the aliens. She and the human step into a conversation in which she tells him - "...they are not changing me, they are changing me back." The door to the cell shut, and the human realizes that all is lost. The end of the story is followed by a grievous scream from the human.
The episode may have lasted up to about 45 minutes.
I also faintly remember that the human somehow got out of the cell through an air vent of some sorts. I remember that he used some sort of crystal he found in the cell as a weapon, hid it in the cell walls.
He was also wounded as I remember, and before the female human came into the cell, there were other male humans (soldiers/troopers). I don't remember what happened to them or if they were in the same cell as the first human (officer).