Abby and Zara strive to use their powers for good but it's clear they are damaging reality - and allowing monstrous creatures to bleed through from beyond. The Doctor knows he can only save the universe by destroying his friends. But just how much might he be willing to sacrifice if there's a chance to save them?
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 — and ones like Castrovalva, where the regeneration sequence was replayed — are not included.
Regeneration is usually considered a biologic process exclusive to Time Lords and a few other species. However, some consider Regeneration and The Eclipse of the Korven to also be "regeneration stories", though neither describes anything close to a biological process. Korven is a particularly hard one to include in this list, because K9 is said to "regenerate" but is largely physically unchanged.