And so, after an episode that's all about messing with people's heads and establishing the premise, 'The Singing Sands' settles in and does exactly what a story like this is supposed to do. Lucarotti is telling an exciting story of a journey through foreign lands, and that means the heroes have to overcome obstacles rooted in the cultural and geographical lore of the countries they're traveling through. Which means that the caravan has to go through the Gobi desert, which in turn means that the big drama is going to come from sandstorms and thirst. It's not rude to point out that these are old standards--a story that didn't hit these beats wouldn't feel like it was doing its job.
Likewise, Tegana is playing exactly the kind of character that needs to be in a classic adventure story; he's villainous enough that we can all lustily boo his schemes and machinations, but not so effective that he's actually disturbing. His plan to murder everyone in the caravan by cutting their water skins hits just the right sweet spot. It's potentially deadly, but not actually scary. (Which is actually one of the criticisms of the story--that Tegana is such an obvious mustache-twirling villain that the heroes seem like idiots for trusting him as much as they do. But at least at this point, it's clear that the TARDIS crew don't trust him at all, and Mark Eden does a good job of playing the part ambiguously enough that you can at least make a case that he doesn't so much trust Tegana as tolerate him.)
That said, it is an episode that does pretty much what it says on the tin, no more and no less. It's an adventure story for children, with none of the moral ambiguity or interpersonal drama of the previous fourteen episodes. Even worse, the Doctor is almost entirely missing from the story. As great as Ian, Barbara and Susan are (and at this point, I'm fully convinced that this is the best TARDIS crew ever), they need the Doctor to play off of. So does everybody else. The big emotional conflict is between Polo and the Doctor, even though Tegana is the ostensible villain; with one of them sidelined for the episode, we're left with the Boy's Own storyline for a full thirty minutes. And while that's not bad, it's not as good as it was or as it's going to be.