"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...