It's a little frustrating that this is an episode that doesn't know what the series' strengths are at this point. They have a great regular cast, all of whom are bringing their A-games week in and week out. They have a great guest cast who are really engaging with their material. They have some really good emotional material here to work with. And we just don't get enough of the real conflicts. Tegana is lying when he says that "bandits" prevented him from getting back to the caravan with life-saving water, and everybody knows it. But that scene is cut tragically short. Likewise, the conflict between Polo and the Doctor is really great stuff, with Polo as a sympathetic antagonist and a really fascinating ethical dilemma...but we get one snippy exchange in the whole episode.
Instead, we get a lengthy sequence where Ping-Cho tells the story of the Hashishin, which...look, on the one hand, I have pangs of nostalgia for this scene. Years ago, as a kid, I learned the history of the Hashishin from the Target novelization of this story. It's still got a place on my bookshelf, years after I let the rest of the Target books go in order to conserve shelf space. (The only other survivor was 'Remembrance of the Daleks', although I've built it back up a bit since then.) And it's performed beautifully by Zienia Merton. So with all that, I can't say that it's a bad scene, I really really can't.
What I can say, though, is that it doesn't really fit in with the tone of the episode that surrounds it and that it kills the momentum of the episode. And this is not an episode that can afford a five-minute stall, not with as little plot as it has. We've had a five-minute sequence involving the explanation of condensation, and now we're getting a history lesson. Even for a series that at this point has an educational mandate, this is draggy.
Luckily, Tegana is on hand to at least nudge the plot along by having Barbara kidnapped. It's really more of a set-up for next episode's action than anything else, but at least Jacqueline Hill gives it her all the way she always does. (Have I mentioned yet that the regular cast brings their A-game every week? Oh, yes. Right there, three paragraphs ago. Carry on.) This episode is definitely a bit of a longueur, but it's hard to get through a seven-parter without one. And as longueurs go, at least this one has the tale of the Hashishin.