After last week's cliffhanger, I honestly didn't have high hopes for this week's episode of Doctor Who.

As I detailed in my review, Oxygen was a fun story. A little on the nose with its social commentary, but highly enjoyable. However, the cliffhanger reveal that the Doctor was still blind was lackluster and unwarranted. The expectation, then, was that this week was going to be spent dealing with it and, hopefully, moving on. Instead we got what is, apart from the unnecessary blind-Doctor story-arc, possibly my favorite episode of this series so far. Mixing in high-concept sci-fi with Dan Brown-ish creepy priests, we get a story that genuinely keeps the audience guessing throughout, and then nailed the landing with the eventual reveal. Colour me impressed.

In the story, the, now blind, Doctor is moping about outside the vault. He flashes back to a time on a planet when Missy was about to be killed by a race that specializes in execution. He is to be the one to flip the switch, so to speak. Obviously he has mixed feelings about this. As he sits reminiscing, a message comes to him titled 'Extremis'. Suddenly he is met by representatives from the Vatican… and then the Pope himself. They are calling on the Doctor to help them with an ancient document called 'Veritas'. Anyone who reads this document commits suicide within a day. The Doctor recruits Bill and and Nardole to help, and the two of them take off for Vatican City. Whilst there, portals start appearing in the walls, strange hooded creatures start following them around, and the nature of 'Veritas' turns out to be a lot more severe than anyone anticipated…

"The whole thing feels positively Philip K. Dick"

What's particularly striking about this episode is how bleak it is. The tone is so maudlin. When mixed in with the sci-fi concepts being discussed about empiricism, memory, and knowledge of the self, the whole thing feels positively Philip K. Dick. This might sound heavy in a family show, and it is, but it's executed brilliantly. The trickle drip feed of information is perfectly paced, and the short diversions to CERN and the Pentagon create a 'What?' factor, that we haven't seen done this well since Series Three's Blink. What's exciting is that this show will likely introduce a young generation of television watchers to high-concept sci-fi, which is a thrilling thought.

It's a shame that we're still saddled with the 'the Doctor is blind' story thread, though. It's just not necessary. This episode would have worked just as well, if not better, without it. What's more, it has given Steven Moffat the excuse to bring back the, equally unnecessary, sonic sunglasses. With any luck they, as with the blindness, will not be around too long.

The scenes between Michelle Gomez and Peter Capaldi in this episode deserve a special mention. Capaldi is a Class A actor with such a strong presence that he elevates everyone around him. Gomez is one of those British actresses who has been quietly excelling in everything she's done for years (particularly Green Wing), and seeing her now in a role that shows her off to the wider world is just marvelous. When she and Capaldi are on screen together, the sparks fly and fireworks go off. What's great is Gomez is allowed to play the role so much straighter than normal. She's fun when she's being wild and crazy, but she smolders when she brings the energy down. Combine that with Capaldi's unmistakable gravitas, and you have yourself a great set of scenes.

Overall then, this episode may be one of the best, if not the best, of the series so far. It has set up a story which promises an excellent further payoff to this week's teeing up. Next week… pyramids.