Planet of the Boundary

The planet of the Boundary was the inhabitable but uninhabited world where the Boundary stood. Seeking to pass through the boundary, human refugees from the Cyber-Wars once founded a settlement near the Boundary, under the leadership of Ko Sharmus, erstwhile member of the Alliance.

The Thirteenth Doctor, Ryan Sinclair, and Ethan arrived on the planet in a stolen Cyber-shuttle. Ko Sharmus showed them the Boundary, which appeared as a rippling sheet of energy. It unexpectedly cleared, and the group saw the capital city of Gallifrey on the other side. The Master leaped through the Boundary and informed the Doctor that "everything [was] about to change forever". (TV: Ascension of the Cybermen)