Peter Capaldi will end his tenure as the Twelfth Doctor on Doctor Who in the 2017 Christmas Special. Capaldi just finished filming the episode, which means at least the production part of the Doctor Who experience is now behind him.

Looking back on his three-season run as the Doctor, which episode does Capaldi see as the one that most defined his Doctor? Den of Geek was part of a group of reporters at San Diego Comic Con today who were given Capaldi’s answer…

I think “Listen,” in the first season because it’s the first time we clearly see this kind of obsessive quality about him. Because, in that episode, he has a compulsion that there’s a creature that exists that you can’t see, and he sets off to find it, even though you can’t see it and there’s no evidence of it, and he’s absolutely committed to that idea.

“Listen,” written by showrunner Steven Moffat and directed by Douglas Mackinnon, was Season 8, Episode 4 of the show. It sees Clara struggling to balance her relationship with Danny Pink with her travels with the Doctor. For his part, the Twelfth Doctor spends his time hunting down a creature with the perfect ability to hide.

Why does Capaldi find this episode so important for the Twelfth Doctor? Capaldi says…