In the second season episode The Dalek Invasion of Earth, the Doctor chooses to leave his granddaughter, Susan, behind with the man she falls in love with, David, in order to let her sprout her own wings and find her way through life. This episode gave fans one of the most memorable lines from not only the First Doctor, but in all of Doctor Who history. "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."