Doorway to Hell is set after the Doctor and the Master's "affair with the Daleks and Draconians", referring to Frontier in Space, the last appearance of Roger Delgado as the Master prior to his death.
The fact that Roger Delgado's Master is shown regenerating either retroactively establishes that the decayed Master seen in TV: The Deadly Assassin and The Keeper of Traken is not the same incarnation as that played on TV by Roger Delgado, as had previously been implied in The Deadly Assassin, or that this was simply the botched regeneration into Pratt/Beevers' Master, which burnt his body to a crisp. This seemingly contradicts PROSE: Legacy of the Daleks, but, since we are not shown his new form, he could have possibly somehow kept the same Delgado body.
Original print details
(Publication with page count and closing captions)
The Doctor is from far in the Master's future, and says his age is four billion. Ohila had previously mentioned the Doctor had spent four and a half billion years inside the confession dial. (TV: Hell Bent) However, the Doctor would later describe his age as 2,000. (TV: Smile, et. al)
For the purposes of this list, a "regeneration story" is one in which a regeneration is actually and initially depicted. For this reason The War Games is not included below, even though it is commonly thought of as a "regeneration story". It doesn't actually include a clear scene of regeneration, and the preponderance of stories in other media confirms that the Second Doctor did not regenerate at the end of it.
Additionally, immediate post-regeneration stories, like the 2005 Children in Need Special Born Again — and ones like Castrovalva, where the regeneration sequence was replayed — are not included.
