After defeating the Ice Warriors, the TARDIS crew materializes on Earth in the near future where the Doctor is immediately mistaken for someone else and people start taking pot shots at him. The Doctor soon discovers that he closely resembles a man named Salamander who appears on the surface to be a benefactor to the Earth, but is slowly amassing more and more power. Salamander is The Enemy of the World.
Patrick Troughton gets to play a double role in this story. Hartnell did something similar in The Massacre of St. Bartholomew's Eve although it was never quite clear if the secondary role actually was the other character or the Doctor impersonating the other character (although the latter is implied). In this story, Troughton plays the Doctor, Salamander, and the Doctor impersonating Salamander. :) Sadly, only one episode of this six part story exists. Again, thanks to telesnaps and audio recordings, Loose Cannon can provide us with a reconstruction of this story.
This story is generally considered to be one of Troughton's best. I was familiar with this story going into it, but didn't know what to expect beyond a few brief clips I'd seen. Wow... I was completely drawn into the story even though it's a very nontraditional story for Doctor Who. I'm not sure how authentic Troughton's Yucatan accent was, but overall, the story worked pretty well. It took a bit of an odd turn toward the end of the story when we meet the "survivors" living in an underground bunker, but that was mostly explained right at the end. The last couple of episodes had me literally on the edge of my seat. :) So far, season 5 is turning out to be really good.
With 5 of the 6 episodes of this story missing, our total missing epsiode count for season 5 climbs to 12 and Troughton's totals are: 3 missing stories, 7 incomplete stories, and 40 missing epsiodes. Next time, the TARDIS materializes in the London subway where the Yeti have invaded in The Web of Fear.