Extremis is the first episode in first multi-part story of this season, and boy what a set up it was. Ostensibly about The Doctor, Bill, and Nardole being brought into the Vatican to examine a text called the Veritas, a book that everyone who had read has subsequently killed themselves.

The big twist, which was wonderfully written by showrunner Steven Moffat, is that actually almost this entire episode is set inside a simulation and that nothing that happened is real. Instead it is all a test invasion from the unnamed alien race to work out the best strategy for invading the Earth, and dealing with the biggest threats to them, such as The Doctor. It’s not surprising that this was a Steven Moffat penned episode because this is such a brilliant concept for an alien, and their way of invading. It’s a concept that belongs to be held up alongside the ideas of the Weeping Angels or The Silence as genius monsters that Moffat has created.

The ending of the episode sees the simulation of The Doctor use the information that he’s been recording through his sonic glasses (still one of the silliest Doctor Who inventions, but that works a little better with his blindness) to warn the real version of himself about the coming invasion. This was the moment that the major plot of the episode had been building towards. It is clearly a set up episode for what is to come, and that could feel a little dull, but it is so unexpected and different for an episode of Doctor Who that it really works.

But this wasn’t the only big plot point in the episode. Right from the start we had big reveals about the vault that The Doctor has been guarding. And it’s Missy, the current incarnation of The Master. She’s been sentenced to death with The Doctor to act as her executioner. There are a lot of mentions of River Song, and her time spent with The Doctor on Darillium, when it is revealed that she sent Nardole to stop The Doctor from actually executing Missy. The Doctor makes the decision not to kill Missy, but he will still imprison and guard her for 1000 years as promised. It was a surprise to get answers this early in the season, but it is clear that Missy is going to play a big part in the war that is to come. It would not be surprising if Missy was playing The Doctor, but as it stands they may well be on the same side.

This was another great episode of Doctor Who, probably the strongest of a very strong season. It was certainly a risk of an episode, but it really paid off, as it has in episodes such as Heaven Sent. Again Capaldi shines, and Pearl Mackie’s Bill is proving to be a great companion. But it was great to see Michelle Gomez back as Missy, and this was a stronger episode for Matt Lucas’ Nardole as well. The next episodes in this multi-parter should be exciting.