"Britannia" is set in 43AD as the Roman Imperial Army, determined and terrified in equal measure, returns to crush the Celtic heart of Britannia, a mysterious land ruled by wild warrior women and powerful Druids who can channel the powerful forces of the underworld. Arch-rivals Kerra (Kelly Reilly) and Antedia (Zoe Wanamaker) must set aside their differences to combat the upcoming Roman onslaught, led by the towering figure of Aulus Plautius (David Morrissey).