Nobody has ever - or probably ever will - explained the wibbly-wobbly, timey-wimey, er, big ball of stuff that is the bootstrap paradox in such a rock ‘n’ roll way like Capaldi’s Doctor. Although, let’s be honest, most of us still did Google it anyway.
Capaldi then dons the coolest equipment associated with his Doctor (no, not the distinctly uncool sonic sunglasses; his electric guitar). He plays the opening bars of Beethoven's Fifth Symphony before leading into his very own guitar riff of the Doctor Who theme. The fact the producers didn't continue to use the rock-inspired arrangement is as baffling as the bootstrap paradox itself.
Nevertheless, given the wonders of modern technology, we can always revisit that mini medley, and also enjoy, or you may even dismay, the other distinct and compelling character medley of the Twelfth Doctor in this episode.
His practically nonchalant reaction to O'Donnell's death at the hands of the Fisher King shows how much of an emotional detachment he exhibits in such highly-charged situations, especially when compared to his two previous incarnations.
There's the Doctor's famed deceptive streak, the good, the bad and the ugly. First the bad and the ugly: when Bennett accuses the Doctor of "wanting to test his theory" on whether O'Donnell might of survived having not seen her ghost. The good: tricking the Fisher King into thinking he's removed the glyphs from the ship, thus saving the world yet again. No wonder UNIT appointed him President of Earth.
Then there's that casually arrogant look into the camera at the end as if to say: "Well, who else?"