The episode – which reunites the Doctor with Sarah Jane Smith and Jo Grant (former Third Doctor companion played by Katy Manning) – features a moment in which Clyde Lander (Daniel Anthony) quizzes the Doctor on how many times he can change.
Clyde, Luke and Rani all encountered the Tenth Doctor in David Tennant’s last recorded performance as the Time Lord in The Sarah Jane Adventures Series 3 story The Wedding of Sarah Jane Smith, which co-starred Nigel Havers doing his now familiar aging smoothy role…
While the Doctor and Clyde Langer… are in the process of outwitting spooky vulture undertakers the Shansheeth, Clyde asks how many times he can regenerate.
The Doctor indicates that there is no limit. The action continues.
As we’v already seen, the 12 regenerations (13 body) limit imposed by the Time Lords is arbitrary. They offered the Master a new cycle of regenerations in The Five Doctors, and as recently as Last of the Time Lords, we found that the insane Gallifreyan had been “resurrected”. Indeed, it seems that even Rassilon himself was reawakened to lead the Time Lords against the Daleks (The End of Time).
So all of this should come as no surprise.