Dog dimension

The "dog dimension" (so dubbed by the Thirteenth Doctor) was a parallel universe where "dogs were people and people were dogs".

The Doctor and Team TARDIS were once trapped in the dog dimension. Fearing for the fate of the universe if she couldn't escape soon, the Doctor wrote a book, which she managed to send back through to N-Space, containing a brief account of her predicament and a set of challenges and instructions intended to allow whoever found the book to temporarily take on the mantle of "the Doctor". (PROSE: Meet the Fam)