Doctor Who has had pretty much the same formula since it debuted in 1963, making it all the more notable when an episode deviates from the show's established blueprint. On Nov. 28, 2015, one of those outliers aired for the first time. It was so unique that it was tricky to judge where the episode belonged among the pantheon of iconic Doctor Who episodes, but it has gradually become accepted as the finest effort the long-running British sci-fi show has ever produced.

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