"To protect a muddy little hillside, you doomed your whole world!"

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...