During The Dalek Invasion of Earth, Susan (the Doctor's Granddaughter) falls in love with the freedom fighter David Campbell.
The Doctor realises that Susan would never leave him of her own free will because she thinks him dependent on her.
Rather than let her choose to stay with him or with David, he forces her hand and locks her out of the TARDIS, and makes a very nice farewell speech to her: "One day, I shall come back. Yes I shall come back. Until then there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine."
So what did Susan, or rather the actress Carole Ann Ford do next?
Well she had a very different, and far more grown up role (and a northern accent!), in an episode of Public Eye: The Morning Wasn't So Hot (1965).
Here is a clip below, and for a short time only the full episode can be see (with an interview with Alfred Burke who played Frank Marker) on the Network DVD site here.
Public Eye ran from 1965-1975, an excellent series - "Marker isn't a glamorous detective and he doesn't get glamorous cases—he doesn't even get glamorous girls. What he does get is people who are in trouble—the sort of trouble you can't go to the police about, even if you are innocent."