The Doctor takes Bill and Nardole back to 2nd century Scotland to learn the fate of the 'lost' Ninth Legion of the Imperial Roman Army. 5,000 soldiers vanished without explanation - how?

The search for the truth leads the Doctor and his friends into a deadly mystery. Who is the Guardian of the Gate? What nightmare creature roams the wildlands, darkening the sky and destroying all in its path? A threat from another dimension has been unleashed on the Earth, and only a terrible sacrifice can put things right...