A mysterious Doctor was a time traveler whose visits were known of by the inhabitants of the 40th Century. Apparently, he had visited the late 30th Century. ("The Time Machine: A Sequel") He is an alien, belonging to a race that is very old and very powerful, and mastered time travel centuries before humanity even arose.
The Doctor's TARDIS time machine appeared at some point alongside The Time Machine in a warehouse of illegal time-travel devices. ("Star Trek - DTI: Watching the Clock")
- This mysterious Doctor is implied to be the very same Doctor from the television series, Doctor Who.