The Doctor and Susan in “The Dalek Invasion Of Earth”

During all the years I've been taking care of you, you in return have been taking care of me.

Susan

Oh, Grandfather, I belong with you!

Not any longer, Susan. You are still my grandchild and always will be. But now, you're a woman too. I want you to belong somewhere, to have roots of your own. With David, you will be able to find those roots and live normally like any woman should do. Believe me, my dear, your future lies with David and not with a silly old buffer like me. 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. Goodbye, Susan. Goodbye, my dear. [He dematerialises the TARDIS, which vanishes. David and Susan leave, Susan dropping her key onto the floor.]