Doctor Who Unlocked: The Return of Doctor Mysterio

The episode is based on the idea of Superman's alter-ego of Clark Kent and indeed Superman images appear in the episode. Superman was created by Jerry Siegel and Joe Shuster and permission for the images was given by Jerry Siegel's family. The creators are then namechecked straight after the credits with Mr. Brock saying that any questions "can be handled by Miss Shuster and Miss Sigel".

The episode is set in New York but most of the external New York street shots were actually filmed in Nu Boyana Film Studios in Sofia, Bulgaria.

On the street you might be able to make out the name of the film being shown at the cinema- it's Mind of Evil, named after the Third Doctor story of that name. Also on the street is Joe's Pizzas which is where Peter Parker worked in Spider-Man 2.

The Doctor references the famous Spider-Man line "With great power comes great responsibility".

The Doctor mentions how he always has invasions at Christmas. This references the various Christmas Specials such as The Christmas Invasion and The Runaway Bride where he has fought off Christmas Day invasions.

This episode follows on from The Husbands of River Song in various ways. Scratch in Husbands has similar physical characteristics and said "I represent the Shoal of the Winter Harmony" which is very similar to the company name Harmony Shoal.

Nardole returns, now with a full body again. In The Husbands of River Song his head was attached to Hydroflax but mention is made that the Doctor reassembled him.

Nardole tells Grant and Lucy about River Song and a night that lasted 24 years- he is referencing the end of The Husbands of River Song where the Doctor takes River to the Singing Towers of Darillium. He also mentions that River died in a library a long time ago. This happened in Forest of the Dead but was a long time ago for the Doctor.

The UNIT soldiers dealing with Harmony Shoal mention Osgood, the British UNIT scientist.

The episode title The Return of Doctor Mysterio is explained in the episode from what Grant calls the Doctor. In Mexico the show is known as 'Doctor Misterio' which Peter Capaldi discovered during the 2014 world tour, much to his delight. Writer Steven Moffat recalled that delight and used in for the episode.