Doctor Who: The Ark in Space (1975) written by Robert Holmes and directed by Rodney Bennett
The Fourth Doctor (Tom Baker) materializes the TARDIS on a space station orbiting the Earth in the far distant future. The Doctor explores with his companions Sarah Jane (Elisabeth Sladen) and Harry (Ian Marter), only to discover the station is deserted (except for thousands of people in cryogenic sleep) and apparently sabotaged. The Doctor makes some repairs which activates the security, causing complications. Also, the repairs start the revival sequence for the humans on board. They've been set aside to avoid an apocalyptic disaster on Earth, though they should have been revived centuries ago. The problem is the Wirm, a parasitic intergalactic species that landed on the station a long time ago and is now attempting to take over all the humans. Can the Doctor and his companions save humanity?
The story has a slow start (the first of the four episodes is almost dispensable) but picks up speed as it goes along. One of the revived humans, the station's commander who is nicknamed Noah (hence the "ark"), gets infected and becomes the main villain, though he struggles to maintain his humanity even as he slowly transforms into a Wirm. The special effects are low-budget (the aliens are actors covered in green bubble wrap!) and are, at best, less convincing to contemporary eyes. The commander's struggles against transformation are overacted and also less satisfying (and there's a weird edit that's explained in the supplemental material on the DVD). Even so, his character has an interesting arc that isn't as obvious as it seems. The ultimate resolution isn't surprising but is satisfying.
Tom Baker is in top form as the Doctor. He combines the intelligence and arrogance with a sense of wonder and admiration of humans that makes him an inspirational hero. He hardly ever resolves his problems with fighting (either fisticuffs or laser guns). Often the show lets the companions do that stuff. Baker is charming and odd, with plenty of eccentricity and cleverness to see the situation through.
Recommended, though be sure to make allowances for 1970s BBC visual effects.