5.2 - The Beast Below

From wikipedia

In the distant future, the Doctor and Amy arrive on the Starship UK, a colony spaceship containing the population of the United Kingdom who has left the planet to escape deadly solar flares. They come across a crying girl, Mandy, whom all the other adults aboard consciously ignore. The Doctor, having discovered that the ship does not seem to be powered by normal engines, tells Amy to follow Mandy while he explores the engine room. There, he finds the engine controls to be a false front, and encounters a masked woman called Liz 10, who also is aware of the ship's oddness and of the Doctor's identity.

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