The Doctor and Clara Oswald receive a message from Reggie, a character from a previous episode. Reggie has a tattoo that is counting down but doesn't know how or why he has it. The three go off in search of how he got it by retracing his steps and they discover a hidden street which looks like it is close to M.I.6. Headquarters in Vauxhall Cross, London.
They discover that Ashildr is the Mayoress of the area and is giving sanctuary to refugees and migrants. The tatoo on Reggie is because he has murdered someone and that this is his punishment. The Doctor and Clara investigate the murder and during the investigation, Clara gets the sentance past onto her from Reggie which is possible.
They discovers there's been no crime, in fact the murder victim was using as pretence to hiding. Ashildr reveals she can't turn off the countdown and so faces the raven bravely who kills her. The Doctor decides to face the monster behind the sentance and travels through a portal.