In a welcome return to a sci-fi adventure story, The Keys of Marinus catapults our travellers to the planet Marinus. It's a plot straight out of Dungeons and Dragons - a basic collection quest with the fate of the world at stake. The adventurers must find five parts of a key to save the world of Marinus from the evil Voords. Written by Terry Nation (as in The Daleks writer), the Voords were ostensibly the first in a long series of attempts to try and create villains that were just as popular as the Daleks. Predictably, these attempts generally failed.
Overall, however, The Keys of Marinus is a success. The most notable oddity is the absence of the Doctor for an entire two episodes, as William Hartnell disappeared on holiday. (This was apparently not strange at the time, especially when considering the first season of the show ran for the entire year, and held 48 episodes in total). It speaks volumes about Hartnell and the series overall that he's barely missed.
We follow Ian and Barbara with such ease that the emotional reunion an hour later feels bizarre. Barbara is slowly becoming the most interesting character out of the main troupe. Jacqueline Hill often surprises with actual acting, as opposed to 'THIS IS THE 1960'S AND I'M BRITISH' acting. Ian does his best as an English action hero, but he can't quite shake the daggy dad syndrome. Adorably, he swans about in an oriental shirt from his Marco Polo adventure. Like your mid-life uncle who won't stop wearing a Hawaiian shirt after his tropical holiday, so Ian seems apparently taken with his last adventure.
Also notable is the structure Nation employs here, which must have been a nightmare for the production team. The keys are scattered around the planet. In order to find them, the team must fling themselves about the place. The end result of this is that each episode is set in an entirely different location with it's own mini-arc. It makes for a fast-moving story, and the six episodes breeze past easily. It's deeply successful as a piece of contemporary television, and should have been replicated more often. Sadly, it was too expensive to sustain, and decades of Doctor Who would pass before the production team shook the idea of getting as much value as possible out of every single set piece. (The Daleks and Marco Polo, both before The Keys of Marinus, prove this - potentially quite brilliant four episode stories stretched out to breaking point).
With all of its episodes in tact, The Keys of Marinus is the first story of the Hartnell era worth worth watching purely because it's good, rather than legendary. There are no Daleks here, no first-time of anything, it's just a Doctor Who story done very well. For this reason alone, it's worth a watch.