The Doctor and Bill land in London 1814, and the episode starts with lots of interesting discussions of the danger of time travel to a time when slavery still exists, as well as the potential to step on a butterfly and alter the future. With respect to the latter, the Doctor says it is just like any other day. And with respect to the past being “a bit more black” than one sees in the movies, the Doctor says “so was Jesus” and adds “history’s a whitewash.”
As they explore a “Frost Fair” on the frozen Thames, a boy steals the Doctor’s sonic screwdriver. On the ice, mysterious lights take the boy – the Doctor just barely managing to grab his sonic screwdriver from the boy’s hand before he is taken through the ice to his death. This leads to Bill asking how many people the Doctor has seen die, and then how many he has killed.
Under the water, the Doctor and Bill find a large creature that has been chained up. A man named Lord Sutcliffe is from a family that has used the creature’s excrement as fuel for generations. He plans to blow up the ice and feed large crowds to the creature, to make more of the fuel, saying it is no different from, and perhaps better than, coal mines in which many people die as well.
The Doctor advocates calm, saying that “Passion fights but reason wins.” But he ends up punching Sutcliffe when he says racist things. Not long after, the Doctor gives a speech, in which he says, among other things, “Human progress isn’t measured by industry, it is measured by the value you place in a human life…”
He also says later, “Only idiots know the answers.” The Doctor asks Bill what the future of her country is worth if it is built on the suffering and imprisonment of the creature. He lets it be her decision (later calling her ‘boss’). So they work to free the creature. As a result, Sutcliffe drowns, and his fortune goes to a street urchin they had met.