Johann Schmidt

Johann Schmidt was the eighth incarnation of the Doctor from an alternative timeline where the Nazis won World War II. In this timeline, the Seventh Doctor fled Colditz Castle after his companion Ace was killed by the Nazis. Immediately afterwards in his personal timeline, he returned to Germany in 1955 where he was shot and killed by Nazi soldiers. The TARDIS was seized by the Nazi authorities. Absconding after his regeneration, he adopted the alias "Johann Schmidt" and later told Elizabeth Klein that he was a collector of sorts who had stolen the Doctor's body.

For the next ten years, he helped political prisoners escape the clutches of the Nazis and disrupted the Third Reich's "ethnic cleansing programmes." Consequently, he attracted the attention of the Nazi authorities but nevertheless maintained his liberty.

In 1965, he was able to manipulate Klein into setting history on its true course by programming the TARDIS to send her back to October 1944, where she met his past self and Ace in Colditz Castle. He was unable to correct the timeline himself as doing so would have involved crossing his own timeline. Not long after meeting Klein, the Seventh Doctor determined the true identity of Johann Schmidt based on the fact that Klein claimed to have returned the TARDIS to 1944 by reference to its flight log. As the Doctor told Ace, the TARDIS does not have a flight log. Klein herself did not become aware of the fact that Schmidt was a future version of the Doctor for another nine years when she was reunited with the Seventh Doctor. (BFA: Colditz, BFA: Klein's Story)

Notably, "Johann Schmidt" is a German variation of the Doctor's more common alias "John Smith". The Seventh Doctor had previously referred to himself as "Johann Schmidt" while impersonating a Nazi Reichsinspektor (NA: Timewyrm: Exodus) whereas, in the proper timeline, the Eighth Doctor used the name while posing as a German spy aboard the British airship R101 in October 1930 (BFA: Storm Warning).

''In this timeline, the Seventh Doctor regenerated for the same reason as his counterpart in the proper timeline. (DW: Doctor Who (1996))''