Doctor Who season 15's shock finale raises many questions, and one harks back to David Tennant's return to the franchise in a 2023 special.

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".