The character of the Doctor was initially shrouded in mystery. All that was known about him in the programme's early days was that he was an eccentric alien traveller of great intelligence who battled injustice while exploring time and space in an unreliable old time machine called the TARDIS. The TARDIS is much larger on the inside than on the outside and, due to a chronic malfunction, stuck in the shape of a 1950s-style British police box.

However, not only did the initially irascible and slightly sinister Doctor quickly mellow into a more compassionate figure, it was eventually revealed that he had been "on the run" from his own people, the Time Lords of the planet Gallifrey.