Series 9 for Doctor Who hasn't been the greatest series for casual fans; with more foreknowledge required, more fan-service, and the main characters being carried over from last series. But for many core fans, it has been outstanding, and quite consistent in quality (apart from minor blips with Episode 5, maybe, Episode 9). This one was mesmerising. It's possibly the best episode of the revival: best of Capaldi's tenure for sure. It's dark and mature, dealing with the theme of grief, it's sad, and it's the most successful experimental episode ever. The whole episode turns out to be an extended monologue for the Doctor, with scares, blood even, and confusing twists and turns. Doctor Who--for fans at least--can surely not get any better than this. The effects are the best they have ever been, emotions are high, and the outcome is mind-blowing (had it not been spoiled in a BBC synopsis...) General audiences may dislike it for the lack of action in the first half, or find it 'confusing'--which you must expect if you're switching on halfway through. For those who can appreciate it, it's an impressive, thematic work of art, with fantastic writing, direction and of course, an awesome performance by Peter Capaldi. I don't think he's my favourite doctor just yet; but he's sure as hell one of the best actors to ever portray the Doctor.