8. Gatiss Pays Tribute To His Old League
The series's second episode, Blood Vessel, is essentially a "bottle episode", taking place almost entirely on board the ship Demeter as it travels from Eastern Europe to England. (A film with the same pitch, Last Voyage Of The Demeter, has been stuck in development hell since 2002 and currently has Trollhunter's André Øvredal attached to direct once it finally launches).
Its nature as a bottle episode is referenced in scenes that have Sister Agatha trapped in her own mind playing chess with Dracula beside a ship-in-a-bottle model of the Demeter.
Throughout the episode there is a mystery about the supposedly sick character none of the crew or passengers are allowed to see, who is travelling in Cabin Number 9 and is revealed to be Sister Agatha, her mind palace chess game occurring inside a giant "9" shaped dungeon.
That number is significant as it is a nod to the mastery of single-location storytelling on fellow BBC series Inside No. 9, in which every story takes place within a house, a room or indeed a cabin with the titular number.
Inside No. 9 is the work of Gatiss's former League Of Gentlemen co-writers and co-stars Steve Pemberton and Reece Shearsmith. While Gatiss has not appeared on that show, he did crop up in the Hitchcock's Rope-inspired episode of Pemberton and Shearsmith's previous series Psychoville, an episode which gave them the idea for No. 9.
So perhaps it's now only a matter of time before he's found behind the door of another number 9 as well.