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…”
The episode thus poses serious questions about what it means to act for good in the world. Bill initially thinks that it is following rules and avoding doing wrong acts, in particular killing people, but also failing to be appropriately traumatized by deaths even if one did not cause them. By the end, the viewer is made to ask whether it might not be better to maximize good and to right wrongs, even if in the process some bad also happens. Thin ice serves as a wonderful metaphor for this. One can tiptoe around trying not to crack the ice. Or one can go into the situation with a diving suit on, expecting that it will get messy as one tries to not merely be passive but to play a positive role. To put it another way, should the Doctor – like a doctor – “first do no harm”? Or should the Doctor (ironically a bit like Sutcliffe, yet in other ways the very opposite) recognize that harm is inevitable, and therefore doing good is better than avoiding evil?