David Tennant is back on Doctor Who, but not as the Tenth Doctor. Tennant became a household name when he first joined Doctor Who in 2005, taking over from Christopher Eccleston as the Tenth Doctor and remaining in the series' lead role until his departure in early 2010. Then-showrunner Russell T Davies exited the series at the same time as Tennant, but are now returning along with Catherine Tate as companion Donna Noble, to see Doctor Who through its 60th anniversary in 2023. However, this isn't another adventure for the Tenth Doctor. Instead, Tennant has become the first actor to play two separate incumbent Doctors, coming back to the sci-fi series as the Time Lord's fourteenth incarnation.
Source：https://www.cbr.com/doctor-who-toymaker-regeneration-red-herring/
