"Billie once changed the whole of television, back in 2005, and now she’s done it again!," added showrunner Russell T Davies. "It’s an honour and a hoot to welcome her back to the TARDIS, but quite how and why and who is a story yet to be told. After 62 years, the Doctor’s adventures are only just beginning."

The 'Doctor Who' Season 2 finale, "The Reality War," just dropped a bombshell of a twist, bringing back a familiar face as the 16th Doctor.

Ncuti Gatwa has left the sci-fi series, "Doctor Who," after two seasons and former franchise co-star Billie Piper is taking his place.