Heaven Sent from Doctor Who season 9 is a masterpiece of storytelling. The visuals and directing are incredible, but the story is what shines. I'm particularly fond of the shepherd boy's tale used to illustrate the billions of years the Doctor spent in the clockwork, puzzle hell running from the veil. The metaphor - a bird sharpens its beak on a granite mountain once every 100 years or so, and the time it takes for the mountain to be worn down from the sharpening of birds beaks...is one second in eternity - has often been used in literature. James Joyce uses it, most notably, in 'A Portrait of the Artist as a Young Man.'