Our son has picked up the phrase “Oh, come on!” to punctuate any plot development that he doesn’t much like. The arrival of the Master warranted one. I don’t agree. The Master turning up is precisely what this story needed.
“Colony in Space” features one of the very few times that we ever get to see the Master putting his schemes together without the Doctor’s involvement. Usually, we meet the character in the narrative at the same time that the Doctor does, but here, he lands on Uxarieus using the forged identity of an Earth government adjudicator, and he casually and delightfully plays the role in order to further his interests. If you were paying attention at the beginning of part one, you may recall that this has something to do with an ancient Doomsday Weapon.
And if you’re putting two and two together, you might figure out that the ruined underground city, where the Doctor and Jo spend most of this episode, houses this infernal device. There are three alien species on this planet: neither the green and brown primitives nor the white, mostly-blind priests can speak, but there’s a third bunch, represented by a shriveled and tiny little guy who lives in the core of some power room, who warns the Doctor to leave and never return, under pain of death. You don’t suppose these are all connected?