10 Saddest Doctor Who Moments

10. Letting Susan Go

This has certainly become a momentous sequence within the history of Doctor Who. Susan Foreman had been there since the very start of the show, introducing us to the many mysteries surrounding the First Doctor. She ultimately caused teachers, Ian Chesterton and Barbara Wright, to stumble upon the TARDIS through their curiosity of Susan's strange behaviour at Coal Hill School.

It was clear throughout her time on the show that her grandfather cared for her dearly, and only wanted the best for Susan. Naturally, once he became aware of her feelings towards David Campbell during the events of The Dalek Invasion of Earth, he needed to step away.

Upon Susan becoming torn between staying with David or caring for her grandfather, the First Doctor made the executive decision to let her go. Her wished for her to grow, and find a life away from him, in which she could develop and flourish as a young woman. Though this was difficult for the First Doctor, he knew it was for the best, promising to one day come back for her.