This adventure should please a few history scholars, though they might not buy into its conclusions. One of the big mysteries of ancient times is the disappearance of the Ninth Legion of elite Roman soldiers in the wilds of Scotland.

History buff Bill has read every book on the subject and wants to know what happened, so the Doctor takes her and Nardole back to ancient Aberdeenshire – only to find a marauding alien, which may provide unorthodox historical answers.

The trio also run into what remains of the Ninth, led by teen soldier Lucius, and a tribe of Picts led by feisty young warrior woman Kar. Can the Doctor unite these sworn enemies to defeat a dangerous alien?