The finale of The Power of the Doctor saw Jodie Whittaker's Doctor regenerate into a surprisingly familiar face in the form of the 10th Doctor David Tennant! The actor was announced as returning back in May 2022 alongside Catherine Tate, reprising her former role as sidekick Donna. At the time showrunner Russell T Davies teased:
Maybe this is a missing story. Or a parallel world. Or a dream, or a trick, or a flashback. The only thing I can confirm is that it’s going to be spectacular, as two of our greatest stars reunite for the battle of a lifetime.
However, with Tennant now revealed to be the 14th Doctor, quite why this has occured and how it will lead into the next Doctor played by Ncuti Gatwa remains a mystery, with Davies commenting:
Three specials have been filmed for the 60th anniversary and are due to be shown in November 2023, with Ncuti Gatwa's first official episode to be broadcast over the following festive period.