The Woman Who Lived

Plot:

England, 1651. A deadly highwayman known only as 'The Knightmare' plagues the dark streets of London, his fire-breathing accomplice by his side. There's something clearly more than human here, and that includes the loot as much as the outlaws. Who are these creatures, and are they enemies to be fought, or friends who might possibly save the Doctor from certain doom on the gallows?