Secret

The Eleventh Doctor noted that he had kept the War Doctor a secret, even from himself. (TV: The Day of the Doctor)

Behind the scenes
In one million BC, the First Doctor told the Tribe of Gum that he was unable to grant them the secret of fire but he was able to present them with something else that would warm them up a bit. This turned out to be Raquel Welch. (NOTVALID: An Unearthly Child: The Unscreened Edition)

The Evening Standard once reported on a "Cyberleader's sordid love secret" in a front-page newspaper article by Sarah Jane Smith. (NOTVALID: Doctor Who? 163)