40. The Enemy of the World

The evil twin scenario is an old plot device. William Hartnell had his twin in The Massacre. And now Patrick Troughton gets his in The Enemy of the World. Unfortunately there is only one episode of this story that still exists. Which is a shame because Troughton gives a tour de force performance as Salamander.

http://en.wikipedia.org/wiki/The_Enemy_of_the_World