Doctor Who: Spearhead from Space (1970) written by Robert Holmes and directed by Derek Martinus
The Third Doctor (Jon Pertwee) makes his debut (in color, no less!) when the Earth is threatened by invasion yet again. A group of objects have fallen to Earth and are not the typical space debris. They are glowing orbs being collected by odd-looking mannequin people (and one cantankerous human farmer). The Doctor is still recovering from his transformation, so roughly the first of the four episodes is more about the local reaction, especially the activities of U.N.I.T., the United Nations Intelligence Taskforce. They are trying to locate and investigate the orbs as well as determine the identity of the Doctor (since he has a new face, he doesn't look like the Second Doctor (Patrick Troughton) anymore). Meanwhile, the baddies are slowly revealing themselves and their nefarious plans.
Slow is probably the key word in describing this story arc. Not much happens in the first episode other than establishing the various characters. The plot moves forward very slowly and the action doesn't pick up until late in episode two. The finale has some wobbly special effects, a bit of a shock after a fairly good, minimalist presentation of the bad guys. Pertwee is more dashing and robust as the Doctor, and not so comical as others before and after him. He may be a little too serious as the Doctor, though his good looks are offset by the flamboyant cape and puffy shirt. He's not my favorite Doctor but he does a good job.