Two of the regular characters - Jack Harkness, and Toshiko Sato - originally appeared on the parent series, Doctor Who (2005). Toshiko (Naoko Mori) was the government forensic pathologist called in to perform the autopsy on the alien corpse in Doctor Who: Aliens of London (2005). Jack (John Barrowman) first appeared in Doctor Who: The Empty Child (2005) and became one of the Doctor's companions. He was last seen in Doctor Who: Journey's End (2008). Eve Myles played a similarly named character on the parent series in the episode, Doctor Who: The Unquiet Dead (2005). It was heavily suggested that the two are related in Doctor Who: The Stolen Earth (2008) when the Doctor and Rose Tyler mention that she resembles the earlier character and she confirms that her family has lived in Cardiff since the 1800s. Freema Agyeman was a series regular in series three of Doctor Who (2005), as the Doctor's companion 'Martha Jones'. Agyeman later appeared in Torchwood: Reset (2008) and the two following episodes as the same character. Peter Capaldi plays 'John Frobisher' in the Children of Earth specials, and also played 'Caecillius' in Doctor Who: The Fires of Pompeii (2008), as well as the Twelfth Doctor from 2013 onwards. Also in Children of Earth, Torchwood: Children of Earth: Day Four (2009). Nicholas Briggs plays Rick Yates. Nicholas is the voice of the Daleks and the Cybermen in Doctor Who (2005)
Captain Jack Harkness:
Torchwood: outside the government, beyond the police. Tracking down alien life on Earth, arming the human race against the future. The twenty-first century is when everything changes. And you gotta be ready.
As this series was released in Spain before Doctor Who, the translation modifies some mayor canon facts in order to make them understandable for the audience. Because of that, the Doctor is always referenced as "Doctor Who" and Cybermen are called Cyborgs. No dubbing actors repeated their roles in both series. Some other modifications were also made when Doctor Who (1963) was first broadcast in Castilian Spanish.