How good was Peter Capaldi in Heaven Sent? Good enough that it’s hard to imagine any other actor pulling it off. Holding your attention for 54 minutes whilst being put (or rather putting yourself) through this grinder of a story. Heaven Sent is probably the Moffat-iest story ever in Doctor Who with the greatest puzzlebox in the series, the grieving mindscape of the Doctor.
The temptation would be to compare Heaven Sent with Tom Baker’s classic The Deadly Assassin, but whereas there was a great story at the heart of Assassin and was experimental primarily because it was companion-less, Heaven Sent was Series 9 second grand experiment, though a great deal more successful than the first.