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 has left his role as the Doctor after playing the iconic character for two series on Doctor Who. At the end of this evening's series finale, Gatwa regenerates into Billie Piper, who previously played the Doctor's companion, Rose Tyler.