This Season 21 story, set between Doctor Who: The Awakening - Episode 132 and Doctor Who: Resurrection of the Daleks (Story 134), stars Peter Davison as the Fifth Doctor, accompanied by Tegan (Janet Fielding) and Turlough (Mark Strickson).
For the first-time viewer, the setup of the Doctor Who series is basic enough: the Doctor is an alien who adventures in time and space in his TARDIS, usually with a human companion or two. I don't think you need to have seen the series before to enjoy this story, but it does help to know that the Time Lords, the Doctor's own people, have certain rules and responsibilities (sadly, never fully defined) where time travel and knowledge of the future is concerned.
It's the far future and the TARDIS crash-lands on Frontios, a planet only marginally suitable for habitation. One of the last human colonies is located here, a paranoid society on the verge of collapse as meteor showers attributed to an unknown enemy, lack of resources and dissent take their toll. Despite his misgivings, the Doctor lets himself be drawn into the colony's problems. But the true threat, and the answer to a lot of questions, is lurking beneath the planet's surface.
For me, this is one of the best Doctor Who stories I've seen. The plot's imaginative and well constructed, the script is excellent, the sets are very atmospheric, there's a strong guest cast, and Peter Davison is excellent as an authoritative, slightly manic Doctor. Tegan is her usual hasty, dumb, compassionate self, but Turlough gets some real development here. He reasons things out and makes discoveries, unexpectedly reveals information vital to the story's resolution, and faces his fears. Both Janet Fielding and Mark Strickson do a really good job here. Also strong are the residents of Frontios: the pompous and rather pathetic Plantagenet, the harsh and authoritarian Brazen, the humane and curious Range and his daughter, the idealistic Norna. All are rounded characters who make a real contribution to the story, well cast and well acted.
This story raises questions with no easy answers. In a situation like that on Frontios, what do you do and who do you trust? How much should people be allowed to know and when should you break the rules? Amongst all the strengths, though, there are a few weaknesses. Some of the directing is a bit clumsy; the Gravis is just a generic villain in a funny suit rather than being truly alien; and there seems no reason for him to have heard of the Doctor before. I don't usually bother with the DVD extras, but the extended/deleted scenes are worth a look here. They give greater prominence to the character Cockerill, and their inclusion in the final cut would have made the story flow better, I think.