You could feel the excitement and anticipation building as we saw the reflections of Peter Capaldi's face in the eel tanks. Finally, we were going to get the link back to The Fires of Pompeii. We would see how Stephen Moffat was going to explain why Caecilius and the 12th Doctor look like the same person.
The reason itself wasn't that interesting or surprising (I do hope it gets developed more as the series continues), but the way it was explained was fantastic! It was so beautiful how the flashbacks to Deep Breath and The Fires of Pompeii were incorporated. Storytelling at its finest!
Who didn't gasp and hold their breath as the 10th Doctor first appeared with that look to Donna? And Donna! The amazing Donna and her heartfelt plea to the Doctor to save someone. Then seeing the 10th Doctor reach out to save Caecilius. It was better than watching that episode the first time.
That entire sequence was my favorite bit of Series 9 so far.
In the last 8 minutes, we also saw more brilliant moments of acting from Peter Capaldi. The introspective Doctor, who we don't get to see that often, elicits such emotion. The monologue about losing people before the flashback sequence and then the questioning of his saving Ashildr . . . I don't know what to say. The only way I can describe it is an emotional sigh.
Finally, the set up for future adventures with Ashildr was exciting. I can't wait to see more of it next week.