The Beast Below is the second episode of the fifth series of the revival Doctor Who, starring Matt Smith as The Eleventh Doctor and Karen Gillam as his companion Amy Pond. The story begins immediately where the last story, The Eleventh Hour, left off.
After an introduction where The Doctor explains to Amy (and any new viewers) how The TARDIS works, he takes her to Spaceship UK, a spaceship where the population of the United Kingdom is flying through space, trying to find a new inhabitable planet. But of course, there is also something nefarious going on: crying children, creepy looking dummies and glasses of water on the ground not doing what they should. The Doctor and Amy do a fair amount of running, and assisted by a new friend who turns out to be Elizabeth the Tenth, they find out that spaceship is powered by an enslaved starwhale, the titular beast below. The Doctor must come to an agonizing decision about what to do, a decision that is averted at the last moment by Amy Pond's intuition about what is really going on.
I have noticed an interesting pattern in Doctor Who series, that each one has one episode where the Doctor is faced by a moral or bioethical dilemma. The Unquiet Dead, New Earth, Planet of the Ood (and to some extent The Fires of Pompeii) and this episode all fit into that pattern. These episodes are often also used to show the companion's influence on the Doctor's thinking, with their youthful perspective seen as a break on what could quickly become a cynical streak in the Doctor. And this episode does indeed fit that pattern. I am glad these episodes don't come too often, because while Doctor Who can be serious, unrelenting efforts to make it Darker and Edgier would undermine the charm of the show.
This episode does a good job of setting up these issues, and reestablishing characters. There are a few places where it seems disjointed: the character of the Eleventh Doctor, both internally and externally has not quite been established, and there are points when the interaction between the characters seems a bit disjointed. However, its still a great story.
This episode also sets up several themes and arcs for the fifth series, including concepts of memory and perception, as well as mysterious cracks in time.
At the end of the episode, the TARDIS' phone rings, with an urgent phone call from Winston Churchill, leading directly into the next episode, Victory of the Daleks.