Stream (The Hollows of Time)

Professor Stream was a time traveller who had a history with the Doctor.

Stream arrived on Earth at some point after the Second World War, opened a small electronics shop in Tottenham Court Road, London, and secured a contract with the Ministry of Defence. He eventually made his fortune in the 1970s through electronics and gathering a following through his written work.

Stream boasted power and capabilities "far greater than any Earth human", which included mind control. He also knew how to operate a TARDIS. (AUDIO: The Hollows of Time)

Behind the scenes
In the original script for The Hollows of Time, Stream was originally intended to be an alias of the Master ("Stream" being a transparent anagram of "Master"). The permissions that Big Finish possessed at the time of adapting The Hollows of Time meant that they could not use the character of the Master in the play; therefore the character of Stream was deliberately left mysterious to reflect this, with the Doctor and Peri specifically stated as looking back on the adventure after their memories have been 'edited' to give them partial amnesia, making it possible that Stream was the Master or the resemblance is just a coincidence.