So Jon Pertwee's run as The Doctor also marks the conversion of the show from black & white to color. The Doctor being exiled on earth was partly a way to explain the Doctor's change of appearance--punishment by the Time Lords--but I suspect it was also practical from a production standpoint. The show had been on for something like 7 years* at that point, so the TARDIS prop was getting pretty beat-up--both the exterior and the interior. And the interior set had never been designed to be shot in color so I suspect stranding the Doctor on earth was a cost effective way to avoid having to build a new set.
Anyhow, it's quite an impressive story. Haven't seen it in awhile and I quite enjoy it. Shot on film and has some very nice production values. We start out with a RADAR observer tracking some "meteors" approaching earth. There's something funny about the meteors. They seem to be in a kind of formation. And they don't burn up in the atmosphere. UNIT sends out teams to search for them.
Meanwhile the TARDIS materializes in a meadow and The Doctor falls out the door, collapsing in the grass.
Then a soldier escorts a woman through security to an office. The woman is Doctor Elizabeth Shaw from Oxford and she's quite annoyed at being shanghaied by Brigadier Alistair Gordon Lethbridge Stuart, the head of UNIT. He tells her about the meteors and explains he needs her scientific brilliance. The Brigadier explains that UNIT was created to deal with strange happenings and confides that the earth has faced alien invasion twice in the past--but they had help. (The invasions were by the Daleks and the Cybermen and the help was The Doctor.) Ms. Shaw is skeptical. During their meeting, the Brigadier is informed that the meteor searchers have found a police box in the middle of a field with an unconscious man. The Brig has a guard put on the police box and heads off to the hospital to see the stranger.
Meanwhile, strange things are afoot at a plastics plant. An inventor is back from America and wants to know why the project he's been working on has suddenly been cancelled. The secretary is strangely quiet and expressionless and has a slight sheen to her skin. The rest of the factory appears to be mostly automated and his old office now has a "OFF LIMITS SECURITY AREA" sign on the door. After a fight with the plant foreman, the man leaves, lingering at the door to his old office for a moment. Another man with a slightly shiny face and blank expression comes in. He seems to have a hypnotic hold on the foreman.
Back at the hospital, the attending physician is confounded by his patient. The lab is apparently playing a joke on him because the X-ray shows 2 hearts. He heads off to get to the bottom of this when he is called by the blood lab, asking why he's playing a joke on *them*, sending them blood that clearly isn't human. A janitor overhears the exchange and tips off the press.
The Brigadier shows up and is disappointed because he assumed the unconscious man would be The Doctor, but he doesn't recognize the patient--who promptly wakes up long enough to greet the Brigadier by name. When the Brig doesn't recognize him, he borrows a mirror and marvels at his new appearance before passing out again.
While all this is happening, a poacher finds one of the meteorites and hides it in a trunk in his shed. His wife is suspicious. And nosy.
Speaking of nosy, the toy developer breaks into the factory and finds his old office full of mannequins in blue jumpsuits and other strange apparatus. While he is looking around, one of the dummies steps down and advances on him. His hand drops open, revealing a kind of gun. The man manages to escape and, hysterical, finds his way to UNIT.
While Ms. Shaw is investigating the shell of one of the meteors that UNIT managed to recover, The Doctor escapes from the hospital. The strange man from the plastic factory attempts to abduct him but he gets away and makes his way to the TARDIS, where he gets shot by UNIT troops.
Luckily it is just a slight wound, but The Doctor is back in his coma. The doctor suspects it is self-induced.
Eventually The Doctor regains consciousness, steals some clothes and an old fashioned car and makes his way to UNIT headquarters (his watch has a TARDIS homing device). He proceeds to help Liz get to the bottom of the Auton invasion (yep, it's Autons if you haven't guessed). At one point he gets Liz to get him the TARDIS key (which the Brigadier has gained but can't use) because he "needs some equipment" and tries to escape, but can't get the TARDIS TO WORK. Next they find out about the poacher and his meteor. So do the Autons. They get there shortly after the poacher's wife finds the meteor and then gives an Auton both barrels of a shotgun--to no effect but the Brig and his men manage to drive it off. Unfortunately on its way back to the factory it manages to kill the toy developer.
The Brig decides he needs support from the regular Army--but the general he's talking to has a meeting to go to. A plastic factory is creating replicas of important people for an exhibit at Madame Tussaud's. By the time the Brigadier has enough evidence to act, the general has been replaced with his Auton copy, store mannequins across the city come alive and start slaughtering people wholesale, and the last "meteor" has found its way to the factory so the Nestene consciousness can create its new form to take over earth.
Without support from the Army, the Brig has to head to the factory with the troops he has. Luckily The Doctor and Liz have managed to build a machine that should "kill" Autons. The Auton general shows up with troops and tries to arrest the Brigadier but The Doctor and Liz "kill" it, with its face turning to a plastic blank when it "dies." The Doctor and Liz head off to fight the Big Boss while the Brig and the troops do battle with the Autons. The Doctor is almost killed when the machine malfunctions but Liz gets it running again and the earth is saved.
In the wrap-up, The Doctor agrees to work for UNIT in exchange for facilities to work on the TARDIS. He begs the Brigadier to let him keep the car he's stolen but can't so he asks for something similar. The Brigadier says he'll see what he can do and we're off to the races.
***
One of the things I enjoy about this story is that the Brigadier is actually a smart complex character. He know's what's going on and is resourceful. He's taking the best steps he can in the face of a potential alien invasion. As the series goes on he becomes more cardboard and boring. He exists to shoot at things ineffectively and have The Doctor explain things to him. But in the early stories he's a formidable character.
*I should note I'm winging all this, so don't take any of it as gospel fact. I may be misremembering some things.