Ep 7. Season 5, Episode 2, "The Beast Below"

"The Beast Below" is the second episode of the fifth season of Doctor Who, featuring the eleventh incarnation of The Doctor played by Matt Smith. The episode follows The Doctor and his new companion, Amy Pond (played by Karen Gillan), as they land on a city-sized spaceship called Starship UK. The ship travels through space carrying the entire population of the United Kingdom. The surface of the Earth was left uninhabitable, and the people were forced to flee into space.

The Doctor and Amy soon discover that something is very wrong on Starship UK. Children are disappearing, and the remaining citizens seem completely unaware. The Doctor and Amy investigate and soon discover a terrifying creature hiding beneath the ship's surface. The creature is revealed to be a massive space whale that has been captured and forced to carry the ship on its back. Its cries of agony are what cause the children to disappear.

In the midst of trying to stop the abuse of the space whale, The Doctor also uncovers a dark secret about the ship's government. The Queen of England (played by Sophie Okonedo) is in charge of the ship, and she has been following a policy of sacrificing some citizens to keep the others in line. The Queen's actions drive a rift between The Doctor and Amy, as The Doctor grapples with the difficult decision of whether or not to allow the Queen's regime to continue for the greater good of the ship's inhabitants.

Overall, "The Beast Below" is a thrilling and thought-provoking episode that delves into issues of morality and sacrifice. The monster-of-the-week formula is effectively used to explore deeper themes of power, authority, and control. This episode introduces many of the key themes and ideas that will become central to Matt Smith's run as The Doctor.