Doctor Who - Spearhead From Space begins with our titular character exiled on Earth and in a hospital. At the same time, meteorites are falling, one of which contains an intelligence that can manufacture Autons, which it plans on using to take over the world. UNIT could really use the Doctor's help in dealing with the invasion, if only they knew where he was.
Spearhead From Space, the first serial of season seven, marks several important firsts. This is the first episode featuring Jon Pertwee as the Doctor, the Time Lords having forced the character's regeneration at the conclusion of the previous season. It is the first episode that Brigadier Lethbridge-Stewart (Nicholas Courtney), whom we'd seen before, is featured as a series regular. It introduces Dr. Liz Shaw (Caroline John) as the new companion. Additionally, the episode has a new title sequence, and marks the first time a Doctor Who serial was made in color. It was the first story aired outside of the series' normal Saturday night time slot.
With so much new, Spearhead from Space is as much a story about the Doctor as it is about the adventure the characters are undertaking. The concept of regeneration is a new one for Lethbridge-Stewart, who has a hard time believing this is the same Doctor he has worked with in the past, albeit in a different body. The process of the Doctor switching bodies is still relatively new to the series, and Lethbridge-Stewart serves as a nice vehicle for the audience to access the concept so vital to the mythology.
This serial also sets up the Doctor's next big arc. He is marooned on Earth at the whims of his fellow Time Lords. The Doctor isn't too pleased with the situation, but makes a deal with UNIT to make things more tenable. This is a character out of his element, which is always fun to explore. Pertwee seems to be the perfect actor to execute this particular plot.