Big Finish Doctor Who/Recap/046 Flip Flop

An experimentation with form, "Flip-Flop" was released as two discs (two episodes per disc) that could be listened to in either order. The story is cleverly constructed so that it makes sense either way.

White Disc
It's Christmas Eve 3090. The Doctor and Mel turn up on a radioactive wasteland that was once the planet Punxatornee. Twenty years before, the planet was devastated by an attack from the Slithergee, who had been refused permission to settle there. With the planet's last resources, the colony has dragooned a professor into building a time machine, with which he will go back twenty years and persuade President Bailey to allow the Slithergee to settle on Punxatornee. But something goes wrong, and Mel gets sucked back in time.

Black Disc
It's Christmas Eve 3090. The Doctor and Mel turn up on the planet Punxatornee, where the human population exist as virtual slaves to the alien Slithergee, and are promptly captured by the planetary police. A pair of humans watches the arrest, and one of them--Stewart--notes to his partner, Reed, that Mel was right about having a time machine. So they bust the well-known terrorists out of jail. The Doctor and Mel are grateful but confused--first the police, and now Stewart and Reed, claim to know them. President Bailey is confused, too--how could the Doctor and Mel have escaped, considering that Stewart and Reed had been shot to death not an hour before?

Tropes

 * Alternate Timeline
 * Christmas Episode
 * Downer Ending: There's no happy ending for the humans on Punxatornee.
 * Have We Met Yet
 * In Spite of a Nail: Punxatornee's fate is pretty bleak, no matter what happens.
 * Merged Reality: Subverted.
 * Mind Probe: Professor Capra's invention in the Black world. (White!Capra built a time machine.)
 * Terminator Twosome: The Black and White teams, from each other's point of view.
 * Theme Naming: Mostly from Its a Wonderful Life.
 * Trapped in The Past: Mel.