They meet a group of colonists, headed by Robert Ashe, who are having such horrible luck, that "unless things improve drastically, [their] colony is in grave danger of starving to death." Since their arrival a year ago, they planted subsistence crops in order to reclaim worn out soil, but the crops shoot up, wither, and then die. They also live in an uneasy truce with the local race of Primitives, whom they give food, not helping their dwindling food supply. Not only that, but two colonists are killed by giant lizards. The Doctor and Jo promptly give their help as usual.
As if they didn't have enough troubles, a detachment from Interplanetary Mining Corporation, headed by the cold-hearted Captain Dent, arrive and claim mineral rights, in conflict with Ashe's claim that Uxarius was classified for colonization. An Adjudicator is sent for, however, they normally favour IMC in disputes. The hot-headed Winton, Ashe's deputy, favours an attack on IMC to drive them out, in contrast to the more diplomatically-minded Ashe. On the side of the IMC, there's the mineralogist Caldwell, who begins to question some of IMC's methods of getting their bottom line. The Adjudicator does come, and guess who it is?
Things heat up between the colonists and IMC, whose role in the colonists suffering may be connected. Then there's Norton, a survivor from another colony attacked by giant lizards and Primitives, whose behaviour in Episode Two becomes definitely suspect.
Colony paints a grim picture of Earth back home, "no room to move, polluted air, not a blade of grass, a government that locks you up if you think for yourself", a place where people don't live like human beings but like battery hens in floating 300 story islands. An unflattering picture is painted of corporations. Dent says in true fascist, corporate style, "What's good for IMC is good for Earth." As for the colonists, he doesn't care the least about their hardships. All he cares is about the profits they'll make in gutting Uxarius of duralinium. It's also an interesting look at the leadership styles and decision-making, Ashe, Winton, and Dent in particular, and why they either succeed or fail.
Other things: Mary Ashe says that "there's no animal life, just a few birds and insects." So, uh..., what exactly does that make birds and insects? Apart from that, Helen Worth stands out as Mary, as does Nicholas Pennell (Winton), Bernard Kay (Caldwell), and John Ringham (Robert Ashe), who also appeared as the ruthless Tlotoxl in Who story The Aztecs and the no-longer available The Smugglers as Blake. Another in-joke was a reference to how the Spanish ambassador was mistaken for the Master, as Roger Delgado (the Master) was himself half-Spanish, half-French.
Some padding is apparent throughout this six-parter, but it's a thoughtful story on the reaction against post-industrial urbanization (the colonists) and the ruthlessness of corporations (IMC).