Ncuti Gatwa regenerated into Billie Piper in the series finale of Doctor Who on Saturday night. He left the role after playing the iconic character for two series. Piper, who previously played the Doctor's assistant, Rose Tyler, said the opportunity to "step back on that Tardis one more time was just something I couldn't refuse".

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!