A Unit of Fear
A few episodes before, the Tardis crew were in Tibet battling an alien known as the 'Great Intelligence', which used robot Yeti to attack humans. As this story begins the Tardis is flying in space, it's captured mid-flight and drawn to the Earth. Unknown to the Doctor, the 'Great Intelligence' is to blame. The Doctor breaks free and lands on Earth in the London Underground system.
It turns out the 'Great Intelligence' has returned because of Professor Travers. 40 years earlier, he was one who helped defeat the Yeti in Tibet. He foolishly reactivated a control device which brought the 'Great Intelligence' back as well. The Doctor is wanted by the 'Great Intelligence' for his brain and the intelligence he posses. Of course revenge is another reason he is being hunted by the Yeti.
As the Doctor, Jamie and Victoria search the London Underground for answers they come upon the military. The Underground has been shut down and London evacuated so the military can destroy the Yeti. With Professor Travers assistance the Doctor is able to convince the Army, lead by Colonel Alistair Gordon Lethbridge-Stewart, he is not to blame for the Yeti attacks.
Attempts to blow up the Yeti fail as it seems the army has a traitor who's informing about the attacks. The Yeti simply use strange web-spraying weapons to cover the explosives and render them harmless. Suspicions run rampant as to who could possibly be the informant, or why they would do so. Everyone from the Doctor to Alistair is suspect, which hampers the investigation.
Ironically, one of the actors playing the Yeti later goes on to play Benton, one of the key members of Unit, (more on that later). When the Doctor deduces the 'Great Intelligence' wants to drain his brain, he alters the machine so he'd be able to drain the 'Intelligence'. However his companions rescue him and the machine is destroyed prematurely and the 'Great Intelligence' escapes.
WHO: Alistair Gordon Lethbridge Stewart
PLANET OF ORIGIN: Earth, 1968
REASON FOR JOINING: He doesn't. He stays on Earth and helps form UNIT -United Nations Intelligence Taskforce, which the Doctor later helps out from time to time.
NEXT: Screams and Screwdrivers