The Doctor (Contents)

One incarnation of the Doctor was the product of a regeneration of an incarnation postdating the Twelfth and Thirteenth Doctors.

Biography
Immediately after their regeneration, the Doctor decided that the attire of their previous incarnation "wo[uld]n't do". The Doctor then chose their new clothing in the TARDIS, sturdy boots and a new hat being amongst the items chosen. After this, the Doctor exclaimed "And that is that". (PROSE: Contents)

Appearance
This incarnation had a "nice" new face which fitted well into place. Their hair was short and their nose was just right. Their legs were firm and their eyes were bright.

Wearing the clothes of their predecessor for a short time and believing them to be "off-the-peg", the Doctor first pondered on wearing a frilly shirt and a scarf before deciding "Think I'm over them" and "A touch too soon" respectively. The Doctor finally chose some sturdy boots, believing them to be handy for the Moon and a "nice" new hat. (PROSE: Contents)

Behind the scenes

 * The gender of this incarnation is not made clear.
 * Although it is not explicitly stated exactly where Contents falls within the Doctor's life it depicts a chest with an item owned by each known incarnation by that point --excluding the War Doctor-- but including a handbag presumably owned by the female incarnation from The Death List, thus implicating this incarnation to be the one that follows her. Russell T Davies also mentioned the handbag in an interview to Radio Times regarding references in his illustrations to Jodie Whittaker's then yet-to-debut Thirteenth Doctor.
 * In TV: The Power of the Doctor the thirteenth incarnation was forcibly regenerated into a version of . Like the incarnation from Contents, the Master's incarnation chose their new costume in the TARDIS by searching through the Doctor's back catalogue of clothing items. Their new outfit included the Doctor's recorder, scarf, and a celery lapel, all mentioned in Contents.
 * The Fourteenth Doctor was revealed in The Power of the Doctor to be portrayed by David Tennant and therefore identical to his tenth incarnation. This Doctor's clothes regenerated along with him and as such he did not choose his new outfit, ruling him out as the incarnation from this story.