Eighth Doctor (Klein's Story), as Johann Schmidt was an alias used by the Doctor
Talk about it here.
In this timeline, the Seventh Doctor fled Colditz Castle after his companion Ace was killed by Feldwebel Kurtz in October 1944. Immediately afterwards in his personal timeline, he returned to Germany in 1955. The TARDIS materialised near a border checkpoint in the West. As he appeared to be carrying a weapon, he was immediately gunned down by Nazi soldiers who were on patrol at the site. After examining his body, they realised that it was in actuality an old umbrella. Although the TARDIS was captured, the Doctor's body seemingly disappeared. 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 and gained possession of the TARDIS key. Klein later described him to the Seventh Doctor as "handsome, in a Gothic sort of way."
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 making contact with her and offering his services as an assistant, claiming that he had taken the Doctor's body and offering her the key to the TARDIS. Jokingly suggesting that it was a shame that they couldn't go back and ask the Doctor how the ship worked, he inspired Klein to discover what she believed to be the TARDIS flight logs so that she could travel back to October 1944, where she would meet 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 time track. Klein's lover Jonas Faber eventually saw through Schmidt's trickery and shot him in the stomach while he and Klein were working on the TARDIS in her laboratory in Berlin. However, Faber was too late as Klein had already decided to disobey his orders and make the trip into the past, only for her actions to undo her own history when her ego caused her to provide the Seventh Doctor with all the information he needed about her history and thus work out how to avert it. Once the proper timeline was restored, this version of the Doctor ceased to exist as did all of the inhabitants of the alternative timeline except for Klein herself. (AUDIO: Klein's Story)
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 did not have a flight log. (AUDIO: Colditz) Klein herself did not become aware of the fact that Schmidt was a future version of the Doctor until 1953, when she was reunited with the Seventh Doctor. (AUDIO: Klein's Story)
Other uses of "Johann Schmidt" Edit
Notably, "Johann Schmidt", also spelt "Johan Schmidt", was 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 the Reichsinspektor General in May 1951 in another alternative timeline (PROSE: Timewyrm: Exodus) and the Sixth Doctor used it when posing as a Brigadier General of the Berlin Fifth Medical Corps during meetings with Adolf Hitler in 1942 and 1945. (PROSE: The Shadow in the Glass) In the proper timeline, the Eighth Doctor would use the name while posing as a German spy aboard the British airship R101 on 5 October 1930. (AUDIO: Storm Warning) The Eleventh Doctor unintentionally used the name when visiting the Lexington Bank, as the bank was expecting an assessment visit from a Doctor Johann Schmidt from Switzerland (PROSE: Borrowed Time).
While claiming to have come from Berlin to have urgent business with Field Marshall Rommel in the Sahara Desert in 1941, the Twelfth Doctor introduced himself as "Doctor Johan Schmidt". (COMIC: The Instruments of War)