Warning: This article contains spoilers for Doctor Who: The Power of the Doctor!Doctor Who revisited the forced regeneration of Patrick Troughton's Second Doctor by making it a key aspect of the Master's plan in The Power of the Doctor. In his attempt to erase the Doctor from history and destroy their legacy, the Master (Sacha Dhawan) forces the Thirteenth Doctor (Jodie Whittaker) to regenerate, triggering a body swap. The concept of forced regeneration was first introduced in The War Games, the final serial of Patrick Troughton's tenure. In that, he was captured by the Time Lords and forced to regenerate into a new body, which was then exiled to Earth as punishment for the Doctor's meddling with the fabric of time.
The final shot of The War Games saw the Second Doctor being plunged into a black void, his new face yet to be decided. The Second Doctor's final regeneration wasn't shown because Jon Pertwee hadn't yet been cast as the Third Doctor. This was similar to the situation facing Chris Chibnall's team as Jodie Whittaker prepared to film her own regeneration, as nobody knew who would ultimately be replacing her. Rather than send the Thirteenth Doctor into the void, however, her forced regeneration was reversed, but this only delayed the inevitable as she later regenerated into someone who looks suspiciously like the Tenth Doctor. The mystery behind the Fourteenth Doctor's appearance will form the plot of the Doctor Who 60th-anniversary specials, and forced regeneration perhaps has had a role to play in this surprising transformation.
Why The Second Doctor Was Forced To Regenerate
The climax of The War Games saw the Second Doctor forced to call on the help of the Time Lords to put a stop to a perverse scheme. A renegade Time Lord known as The War Chief was responsible for forcing soldiers from across all of human history into a never-ending battle for the amusement of the higher beings known as the War Lords. Unable to stop the games alone, the Doctor eventually summoned his people to step in and return the soldiers to their proper times and places. As the Time Lords intervened, the War Chief was executed by the War Lords for his apparent betrayal.
This left the Doctor to face the punishment of the Time Lords for his own catalog of interference in the thread of time. The Doctor defended himself, proudly listing the various evils that he defeated, while chastising the Time Lords for their passive policy of non-intervention. As a punishment, the Doctor is sent to Earth, and the secrets of Doctor Who's time travel are wiped from his memory. When the Doctor protests that he's known on Earth, the Time Lords forcibly regenerate him to change his appearance, essentially meaning that the Third Doctor is living in witness protection.
Forced Regeneration Can Explain David Tennant's Return
The Second Doctor's forced regeneration into the Third was never reversed, and so his life continued as normal until he regenerated into Tom Baker's Fourth Doctor. In The Power of the Doctor, Yaz is able to reverse the process, returning the Master to his dying body, and reinstating the Thirteenth Doctor. It's unclear what the implications of such an action are, and it may be that in reversing the regeneration, Yaz has triggered a degeneration process resulting in the Fourteenth Doctor looking like the Tenth.
David Tennant's Fourteenth Doctor is shocked and a little horrified to realize he's in the body of the Tenth Doctor, suggesting something has gone very wrong. Doctor Who has never fully explored the implications of a forced regeneration, first introduced in Patrick Troughton's finale. In having the Doctor regenerate back into a familiar face, it appears that the show's 60th anniversary will finally delve deeper into this harsh Gallifreyan punishment.