The Doctor, Romana and K9, in search of the fourth segment of the Key to Time, arrive on the planet Tara, an Earth-like world whose landscape bears a striking resemblance to Kent. (Which is probably because the serial was filmed in the grounds of Leeds Castle). Tara is a strange place. Although the inhabitants possess some advanced technology, their society is a mediaeval-style feudal monarchy, dominated by a hierarchical aristocracy and powerful religious orders.
The Doctor and his companions find themselves in the middle of a situation which could be straight from the pages of Anthony Hope's "The Prisoner of Zenda". (Which is probably because the story is a direct if unacknowledged plagiarism of Hope's novel). On the eve of his coronation Prince Reynart, the rightful heir to the Taran throne, is drugged and kidnapped by his ruthlessly ambitious cousin, Count Grendel of Gracht, who plans to seize the crown for himself. If Reynart fails to attend the coronation ceremony he will forfeit his right to succeed. The solution his supporters come up with is to substitute for Reynart not a human double (as in the novel) but an android one, android making being a skill for which the planet is famed. In a nod to Hope's story, however, Romana turns out to be the exact double of Reynart's fiancée, the Princess Strella. (Which is probably because both roles are played by the same actress, Mary Tamm. This development struck me as rather unlikely, given that the Taran Strella and the Gallifreyan Romana are not natives of the same planet or even members of the same species).
Despite the supposed Time Lord prohibition against interfering in the affairs of other planets, the Doctor decides to come to Reynart's assistance and to foil the plans of the evil Grendel. This was the second occasion during Season 16 that he had tangled with the intergalactic aristocracy; in "The Ribos Operation" he had come up against the equally villainous Graff Vinda-K. We learn that the Doctor is a keen swordsman, something which comes in useful on Tara, where the nobility are partial to fighting duels with "electro-swords". (Another piece of plagiarism, in this case from the "light sabres" of "Star Wars", a film which had come out the previous year). We also learn that the Fourth Doctor's recreations include fishing, something also indulged in by the Sixth Doctor, and chess; Izaak Walton and the chess grandmasters Capablanca and Alekhine are added to his long list of celebrity friends from the Earth's past. (Others include Shakespeare, Nelson and Leonardo da Vinci).
Mary Tamm was to leave the series after only one season, saying that she was not happy with the way in which her character was being developed. This serial is a good example of what she meant. In her first serial, "The Ribos Operation", Romana, who is supposed to be the Doctor's intellectual or even his superior, was portrayed as a strong-minded, independent young woman who could think and act for herself. Here she seems more like a damsel in distress whose main function is to get herself kidnapped by the bad guys and then wait to be rescued by the Doctor and K9.
The serial has its good points- Peter Jeffery makes an effective villain as Grendel- but on the whole I felt that Hope's plot did not really lend itself to a science-fiction treatment. Although the novel, first published in the 1890s, had an ostensibly contemporary setting, his fictitious Kingdom of Ruritania has always struck me as a rather feudal, backward-looking place even by the standards of late nineteenth century Europe. If he had set it in the Middle Ages he might have had to change a few details but could still have kept the same basic plot. The Kings, Princes, Princesses, Counts, swordsmen and peasants of this story seem a bit out of place in a society with such futuristic capabilities.