10 Saddest Doctor Who Companion Exits
2. Susan in Dalek Invasion of Earth
The closing scenes of The Dalek Invasion of Earth find the Doctor making the decision to allow Susan to live a normal life, something his ninth incarnation will later state that he can never have. It's notable for being the very first companion exit, and also for being one of the most emotionally charged.
Recognising that Susan has fallen in love with young resistance fighter David, the Doctor locks the doors of the TARDIS and practically forces her to live happily ever after. It's one of William Hartnell's finest performances in the role, and his warmth and melancholy makes it a sweeter moment than it sounds. It's an incredibly strong character moment for the regular cast. Barbara and Ian have humanised the Doctor, and in softening him have allowed him to see that his freewheeling life of adventure will only hold Susan back.
He has to cut the ties and give his blessing to her romance with David. Otherwise, she'll throw away a stable, rewarding future to get back inside the TARDIS and travel to who knows where. It's a beautiful moment that gives a real insight into the Doctor's psychology in a way that the series would rarely attempt until 2005.