It may seem like Ncuti Gatwa's adventures in time and space have been cut short, but the actor says that a 2-season arc was "always the plan."

In the climax of Saturday’s Finale of “ Doctor Who ” Series 15, Gatwa’s Doctor regenerates into Billie Piper, who previously played companion Rose Tyler in Series 1 (2005) and Series 2 (2006). She also made a brief appearance as Rose Tyler in Series 4 (2008) and was in the “Doctor Who” 50th anniversary special in 2013.

Ncuti Gatwa's shock regeneration is followed by an even bigger surprise as the show welcomes back a popular star of TARDIS past. BEWARE! Spoilers ahead!