The Phantom Piper

Following on from the Twelfth Doctor's idea to solve the mystery of the symbol on the TARDIS by finding Alan Turing, Bill Potts prepares for landing by changing into traditional World War II clothes, proud at herself for getting prepared in advance for once. However, when exiting the TARDIS, the Doctor proudly reveals they have actually landed on Athenia, a huge human colony on the Moon. After Bill gets changed back to normal, the Doctor explains to her he never guaranteed they were going to visit the 1940s, while Bill spots the other major inhabitants of Athenia - Galateans. The Doctor describes the backstory of the creation of the Galateans and the ensuing war that followed before explaining that everything seems so peaceful on the lunar base because he erased the timeline, preventing the war from having started, with some help by Chiyoko.