I think 1974's Monster of Peladon is the greenest Doctor Who serial. Feel free to correct me if I'm wrong. But you've got green Alpha Centauri, green Ice Warriors, and the Doctor's wearing a green smoking jacket, green shirt, and green bow tie. Did they plan on having so much green? Part of me thinks not because, although I like this serial, I suspect the Ice Warriors were brought in halfway through when people realised the plot occupying the first three episodes didn't have legs.
The sequel to Curse of Peladon from two years earlier, the Doctor (Jon Pertwee) returns to the planet Peladon hoping to visit his old friend the King, only to find the TARDIS has carried him 50 years further into Peladon's future than his last visit. The former king's daughter, Thalira (Nina Thomas), is on the throne now, trying to keep the peace between the world's powerful religious order headed by Chancellor Ortron (Frank Gatliff) and miners who are protesting the world's membership in an intergalactic union of planets called the "Federation" (no relation to Star Trek's Federation).
There's certainly a lot of potential in this premise. The story aired two years after a 1972 miners strike in the UK and disagreements among the miners in Monster of Peladon about the amount of violence they ought to employ seems like it was specifically meant to address a very real contemporary issue. With Queen Thalira being the first female ruler, one could draw comparisons to Margaret Thatcher and the miner's strike that occurred ten years later. One could even draw comparisons to Brexit with the political conflict between the miners' belief that the Federation is only exploiting Peladon's resources and the belief of the ruling class in the broader benefits of belonging to an intergalactic community.
Maybe writer Brian Hayles decided he bit off more than he could chew--once the conflict is established with the usual business of the Doctor and companion being captured, escaping, pleading for sanity and reason, and concocting desperate plans and solutions, there's not a who lot of insight into the conflict. The Doctor's companion this time is Sarah Jane Smith, played by Elisabeth Sladen, with whom Pertwee didn't seem to have as much chemistry as he had with Katy Manning or Caroline John. Sarah Jane would really shine later, in Tom Baker's second season, but Sladen does give a decent performance in Monster of Peladon--I was particularly impressed by her ability to mime eating.
There's an interesting scene where Sarah Jane tries to encourage the Queen to be more proactive by telling her about Women's Lib, after which the monarch tries to be a little more assertive in controlling the Chancellor. It's one of those moments that makes the recent Christmas special trying to portray Doctor Who as being so very sexist back in the day seem ridiculous.
But all this is pushed aside by the big green men from Mars, the Ice Warriors, led by Azaxyr (Alan Bennion), who I'm apparently not alone in believing was an influence on Darth Vader.
Imagine you only saw the last two episodes of Monster of Peladon, and then imagine no other Star Wars movie but A New Hope exists. In the Doctor Who serial you have this weird, imposing commander who makes a sudden dramatic appearance, wearing a tapered helmet/mask at all times that no-one questions--we also never see him remove the mask--with tinted lenses over the eyes. Then there's also the cape and the odd breathing, his interactions with space royalty and ambassadors--his calling Alpha Centauri "ambassador" while accusing him/her/it of being a traitor, which we know is true, seems like a dry run for Vader's confrontation with Leia at the beginning of A New Hope.
It could be a coincidence, I suppose.
Anyway, with the Ice Warriors there the plot becomes a lot simpler with all the Peladon factions uniting with Alpha Centauri against the Ice Warriors' illegal faction of the Federation and the main issues of the drama in the first three of six episodes are essentially forgotten. Well, it's a fun story in any case.