Big Finish Doctor Who/Recap/S5/E12 The Cold Equations/CC

It's Oliver's first trip in the TARDIS with the Doctor and Steven, and the three of them end up on a spherical space station which appears at first glance to be a giant greenhouse. Upon further inspection, Steven finds that they're orbiting Earth--the Earth of his past and Oliver's future, with its sky clogged full of detritus and debris. Steven knows how dangerous this can be for space traffic, but the aliens who run the station don't seem to ... Or do they?

They're tall, red-skinned humanoids, it turns out, and they know perfectly well what they're doing. They are able to navigate the clutter of debris with surprising skill, except when inconvenient space/time travellers land on their ship and knock it out of its path with their extra mass. The Doctor and co have just done this. Possible disaster is averted when the aliens jettison the excess mass--specifically the TARDIS--and tell the Doctor and crew that they're lucky they weren't sent out along with it.

Despite that, they're nice enough to the Doctor. They're glorified scavengers sitting around above Earth and scouring its surface with robots for anything of use to them. They don't care much for the humans living on the surface (an apathy which includes Steven and Oliver), considering them too primitive to have achieved anything of import. They're also primitive enough to be poisoned without any qualms just because they get in the way of easy scavenging.

This doesn't make the humans happy. They smuggle a woman aboard one of the scavenger craft to come and see why human children are being poisoned. But her extra mass causes just the same problems the TARDIS crew's has: The scavenger module she's on collides with the station at the wrong angle ... and the whole ship is shoved into the path of the orbiting debris and begins to be pounded apart.

The Doctor is separated from Oliver and Steven; while he ends up in the scavenger module with many of the aliens and the human sent from Earth, his companions are stuck in a small area of corridor, broken off from the rest of the ship and with nothing to guide it out of harm's way. Once all attempts to salvage the situation fail, the only option left is to sit and wait for the air to run out ... and, in Steven's case, try to prize Oliver's secret reason for fleeing Earth out of him.

That, at least, works.

Tropes:

Almost Out of Oxygen: A distinct possibility for everybody at one point, but Steven and Oliver get the worst of it.
Artificial Gravity: Generated by the rotation of the circular station the TARDIS crew lands on.
Cold Equation: Brought up by the people in the scavenger module with the Doctor, who's survival is possible if one person draws a short straw. However, as they're trapped in a smaller space with far less air, Steven and Oliver don't even have that option.
Earth That Used to Be Better: A big war wiped out the Earth Empire, and the people living on Earth now are not considered smart enough to be descendants of the people who founded the Empire in the first place. Now they just eek out what life they can make for themselves.
Exactly What I Aimed At: Steven with pieces of space debris, one of which is the TARDIS. Nobody believes at first that he really didn't miss.
Humanoid Aliens
Improvised Microgravity Maneuvering: In this case, with a canister of air they can ill afford to spare, and Steven has to do the required maths in his head.
The Reveal: Oliver's secret from The Perpetual Bond.
Space Is Cold: Steven sticks a hand out into space for a bit; it ends up frostbitten when he pulls it back in.
Invisible to Gaydar: Oliver.
We Are as Mayflies: "Poor Doctor. He loses all of us. [...] He must know it, too. Those he lets travel in his ship, we're all on borrowed time. Our lives must seem to flicker past him--the old man, an exile from his own people. How many has he lost? ... And is that why he never gives up on us?"