10 Longest Pay-Offs In Doctor Who History
2. Twice Upon a Time
In Capaldi's last outing as the Twelfth Doctor, he met up with his first incarnation at the end of his life. With both Doctor's refusing to regenerate, the story follows the pair as they eventually help each other accept their impending regenerations.
While this didn't directly mirror events from The Tenth Planet, it was a beautifully thought out tie-in that made a strong, poignant finale to the Twelfth Doctor's reign which had been built on the foundation of being the Doctor who "saves people."
The themes of death weren't just limited to the Doctors. It also involved a couple of soldiers from the First World War, frozen in a moment, struggling with being able to kill each other.
They're eventually saved when the Christmas Day miracle begins and all soldiers famously put down their weapons and come together. Seeing the good influenced by Twelve, the First Doctor decides he's ready to move on and goes back to his TARDIS.
In this moment, the episode cuts back to the original footage of the regeneration, bringing us full circle to the moment that had taken place 50 years previously.