Torchwood/Recap/S2/E02 Sleeper

A woman named Beth wakes up to a burglary, blacks out, and realizes she's killed one of the thieves. She's taken in by Torchwood, who torture her until the alien inside her shows itself.

There are a lot more aliens like Beth around who fully believe they're human. They'll be activated soon.

The end of the world is nigh, and it seems humanity has only a few moments left to live. Owen suggests they all have sex. "And I thought the end of the world couldn't get any worse," Ianto retorts. The aliens are thwarted in the end.

Tropes:

 * Aborted Arc/What Happened to The Mouse: The last Sleeper implies that there are other cells active on the planet. To date, they have never appeared or even been mentioned again.
 * Suicide By Cop
 * Tomato in The Mirror: