- Actor Allusion: The first time Hardy finds a clue that is a big lead in the case he pulls out a pair of glasses and his voice gets more doctor-ey.
- He also says "No, don't do that" in the beginning of season 2.
- Hey, It's That Guy!: The Doctor and and Prisoner Zero investigate the murder of Sam's son, while Argus Filch deals with a paedo-hunting mob, Gaz thinks he has psychic powers, Lol is now an annoying reporter, Septa Mordane is Danny's grandmother and Grey Worm is Chloe's boyfriend. Oh, and Rory is the local priest.
- Name's the Same: Chloe's boyfriend Dean Thomas. Alfred Enoch, who plays Dean in Harry Potter, shows up at one point as well, to add to the confusion.
- Doctor Who cast reunited: David Tennant (Tenth Doctor), Olivia Colman (Prisoner Zero), Arthur Darvill (Rory Williams), Jonathan Bailey (Psi), Eve Myles (Gwyneth and Gwen Cooper), Meera Syal (Dr Nasreen Chaudhry), Adjoa Andoh (Francine Jones), Peter De Jersey (Androgar) and David Bradley, who played William Hartnell in An Adventure in Space and Time (plus the villain from "Dinosaurs on a Spaceship"), all appear here.
- A few Torchwood cast members are also reunited: As well as Eve Myles, Susan Brown (Bridget Spears), Lucy Gohu (Alive Carter), and Matthew Graville (Doctor from 'End of Days') appear.