Ian Chesterton is a fictional character in the British science fiction television series Doctor Who and a companion of the First Doctor. He was played in the series by William Russell and was one of the members of the programme's first regular cast, appearing in much of the first two seasons from 1963 to 1965. In a film adaptation of one of the serials, Dr. Who and the Daleks (1965), he was played by Roy Castle, but with a very different personality and backstory. Ian appeared in 16 stories and 77 episodes. He later returned for a cameo appearance, played once again by Russell, in the 2022 episode "The Power of the Doctor".

The First Doctor is the original incarnation of the Doctor, the protagonist of the British science fiction television series Doctor Who. He was portrayed by actor William Hartnell in the first three series from 1963 to 1966 and the tenth anniversary story The Three Doctors from 1972 to 1973. The character would occasionally appear in the series after Hartnell's death, most prominently as portrayed by Richard Hurndall in the 1983 multi-doctor special The Five Doctors, and as portrayed by David Bradley in the 2017 Twelfth Doctor episodes "The Doctor Falls" and "Twice Upon a Time" and in the 2022 Thirteenth Doctor episode "The Power of the Doctor", the latter previously having portrayed Hartnell himself in the 2013 biopic An Adventure in Space and Time.