Doctor Who continued its tenth season this week with its third episode, which took the Doctor and Bill to Regency England where something monstrous was waiting under the Thames. Like last week's Smile, it was an episode that was heavy on the throwback feel.
If you are a long-term fan of the show, Thin Ice didn't offer much that you hadn't seen before, but it still worked thanks to the wonderful budding partnership of the Doctor and Bill. Peter Capaldi and Pearl Mackie were able to inject some freshness into this familiar tale.
To complete the throwback feel, the episode contained many a callback to several ideas and concepts from previous episodes, both of the classic and modern era. In particular, Thin Ice tied together the Capaldi era by referencing multiple episodes from the past few years as well as digging right the way back to William Hartnell.
But which of these nods and references stood out most?
The setting of this episode is the 1814 Frost Fair, the last ever time that such an event was held on the frozen surface of the River Thames. The Doctor is a fan of the occasion as he admits to Bill that he has been there "a few times."
He has indeed. Back in 2011's A Good Man Goes To War, River Song tells Rory that the Eleventh Doctor once took her to the 1814 Frost Fair as a birthday treat. Stevie Wonder tagged along and sung for them under London Bridge (though he wasn't aware he had travelled in time). The Twelfth Doctor also offered to take Clara to a Frost Fair back in 2014's The Caretaker.
It's possible, then, that there are three different Doctors running around on the Thames during this point in time!