The plot of "The Keys of Marinus" is too complex to be set out here in any detail. The basic idea is that the First Doctor, his granddaughter Susan and her teachers Ian and Barbara land on the planet Marinus and embark on a quest for the five keys to the Conscience of Marinus, a super-computer responsible for maintaining law and order on the planet. There is a striking similarity to a much later story arc in the series. The entire 16th season is essentially one long serial in which the Fourth Doctor and his companion Romana are charged by the White Guardian (for which read God, the being responsible for maintaining order across the universe) with the task of retrieving the six hidden segments of the Key to Time, an artefact which would give its possessor immense power.
The 16th season comprised six serials and a total of 26 episodes. "The Keys of Marinus" is a single serial comprising only six episodes, in the course of which the Doctor and his companions visit two cities, a jungle, and an icy wasteland in a series of mini-adventures, most of which do not have much connection with the others apart from the fact that one of the keys is retrieved in each. Only towards the end, when Ian is framed for murder and put on trial for his life, with the Doctor as his defence counsel, does the serial take on greater coherence. I mention "the Doctor", but in fact does not appear in the third and fourth episodes. An attempt was made to provide an in-series explanation, but the real reason was that William Hartnell was on holiday. The programme, however, seemed to get on perfectly well without him, strengthening my belief that during this part of the series' history the real heroes were Ian and Barbara rather than the Doctor.
"Doctor Who" episodes at this time were only around 25 minutes long, which was not really long enough to develop the various "mini-adventures" to their full advantage. (I always associated 25-minute and 50-minute television slots, as opposed to 30-minute and 60-minute ones, with the BBC's rivals at ITV, who had to make room for commercial breaks, but "Doctor Who" may have been made with one eye on sales to foreign commercial stations). The parts of the story set in the city of Morphoton and in the icy wastes could, with advantage, have been developed more fully had the serial been extended from six to, say, eight episodes. There is some interesting material here, but the producers allowed themselves insufficient time in which to turn that material into a really interesting story. The serial might also have been more visually attractive, with a greater sense of the culture of an alien world, had the BBC allowed the producers to spend more money; the programme's budget was always notoriously parsimonious. 6/10.