The toll of years put strain on the Doctor's elderly frame. After defeating the Cybermen at the Antarctic Snowcap Station ("The Tenth Planet"), the Doctor was fearfully reluctant to regenerate when he crossed paths with one of his future incarnations ("Twice Upon a Time"), who is similarly reluctant to regenerate. The events of the episode convince both Doctors to go through with their regenerations. After returning to his TARDIS and helping Polly and Ben back inside, the Doctor collapsed and regenerated for the first time.