Bill (the ever-conscious sci-fi fangirl) begins their frigid adventure worrying about the effects of time travel and altering the past. The Doctor responds by humorously teasing her and saying she had a fictitious friend named Pete who began the adventure with them that never worried about altering the past. Since there is not a fellow time traveler named Pete with them, he implies that he must have been erased from history. This is just another reminder of how fantastic their chemistry together is and further makes me believe that this will go down as one of the more memorable Doctor/companion relationships.
Pushing Bill’s character further, we see her first confrontation with watching someone die. This episode showed more of Pearl Mackie’s range as an actress as she conveyed genuine emotion while struggling to deal with the dark complications of the Doctor’s life. Through this, she learns that he has not only witnessed a lot of death, but he has caused a lot, as well. Their relationship has begun to develop as he teaches her that while he despises killing, he must do it at times, and though he can be calloused towards death, it is because he must move on quickly in order to save the lives of others.
The story of this week’s episode revolves around a giant sea creature living under the frozen surface of the river. In the tradition of a classic morality play, the monster of the episode is not actually the creature but its human captors. The business-savvy villains use the guise of the frost fair to lure unsuspecting victims to be eaten by the creature so that its excrement can be used as a cheap alternative to coal. When the Doctor discovers it is not the traditional alien invasion story - but rather an issue of human ethics - he makes Bill choose the outcome. As an alien, it is not his right to decide this issue. As her tutor, he teaches her about those difficult decisions he has made and how issues are not always so clear-cut or black and white.
The end of the episode is yet another tease of what is to come. We are once again shown the door to the vault that the Doctor is guarding, but this time knocking can be heard coming from within. Four knocks to be exact. Perhaps it is just a red herring, but we know John Simm will be reprising his role as the Master, and the last time he returned, it was foreshadowed by a series of four knocks. It is quite possible that the Master/Mistress is being held captive in the vault.