Prologue was the first prequel to the ninth series of Doctor Who, which was released 11 September 2015 on the Doctor Who website and BBC iPlayer.
The Twelfth Doctor has returned to Karn and discusses the nature of friends and enemies with Ohila, the leader of the Sisterhood of Karn. The two discuss an invitation given to the Doctor by an unnamed individual, someone who the Doctor has known for a long time, but has been enemies with for almost all that time.
Ohila doesn’t believe the Doctor when he says he will not visit “him”, and continues to question his reasons. Eventually the Doctor says he will go “soon”, and gives Ohila a confession dial that he says she knows who to give it to. The Doctor comments that he should go meditate somewhere and then leaves the meeting spot.