The Masque of Mandragora
This is a somewhat overlooked, though well produced and entertaining story.
Paul Clarke has written an excellent review that details the plot that an alien energy wants to subjugate the earth in the fifteenth century, and that this power struggle is mirrored by power struggles on earth. Ultimately, the struggle for power is shown as futile here, and the thoughtful and empathetic Giuliano is (naturally and appropriately, I think) portrayed as an ideal ruler. Spoilers follow.
As Mr. Clarke mentions, the story is, despite good supporting acting, the usually strong regulars, and a decent production, somehow unsatisfying. I think perhaps it comes down to the conclusion, and a little too much going unexplained. First, we arent allowed to see the conclusion of the battle between the Doctor and this alien force. While the reasons for this are clear that the Doctor later appears in the guise of that same alien force and leads it to its own destruction we still want a little more there. The attempt to create suspense is admirable, but undermined by the fact that we dont really believe the alien force has destroyed the Doctor and won the day. In other words, the climax weve been building to is cut short in favour of another climax, where the disguised Doctor leads all the brethren (or, what remains of this alien energy helix) back to the same ruins they came from. From there, its the same glowing energy effect weve seen before, and all of them dropping to the ground.
That said, its still a pleasure to watch these regulars, and the Doctor and Sarah have some nice moments, in particular a scene in episode four where the Doctor is doing some calculations and Sarah tiptoes to a chair to read and wait. Its perhaps a tad sexist, but also has to be remembered were talking about a Timelord and a human, here. The conversation that follows is an interesting example of Doctor Who:
Mandragora doesnt conquer in the physical sense, it dominates and controls by helix energy astral force. It takes away from man the only thing worth having.
Which is?
Well, a sense of purpose, what else? The ability granted to every intelligent species to shape its own destiny.
Here we see the typical vagueness of a Doctor Who plot device (astral force) put up against some solid themes and ideas, in the same breath, as the Doctor continues on about a sense of purpose. A sense of purpose is further illustrated in the plot: there are power struggles between the Mandragora energy alien and the Doctor (who wants humanity to have a sense of purpose) and the potential human rulers Count Federica and Guliano (Guliano would allow his people a sense of purpose).
In a sense, this makes the entire fourth episode anti-climactic, as Count Federico is blasted to dust at the end of episode three. And we know the Doctor will go on to defeat the Mandragora energy. Its possible that the best Doctor Who stories combine these kinds of themes and ideas without leaving the viewers scratching their heads about surface details and plot devices (were told at the end simply that the Mandragora has been sent back to square one). That said, this is an entertaining story with some worthwhile moments, such as the Doctor telling Guliano at the end that knowledge will come in time, and that keeping an open mind is the secret. And surely allowing others to create their own purpose, and explore their own pathways, is a part of keeping an open mind? As Sarah says, Poor Guliano, he looks so wistful, but Guliano is actually a brave character he has the courage to be uncertain.