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.

Speaking of Ncuti’s departure, Russell T Davies, Showrunner said: "What a Doctor! Thank you, Ncuti! As his final words say, this has been an absolute joy, and the team in Cardiff and everyone who has worked on this show for the past few years, are so lucky to have been part of Ncuti’s great adventure as he shoots off to stratospheric new heights."

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!