Britannia begins in 43AD as the Roman 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 mysterious forces of the Underworld. Arch Celtic rivals Kerra (Kelly Reilly) and Antedia (Zoë Wanamaker) must face the Roman invasion led by the towering figure of Aulus Plautius (David Morrissey) as it cuts a swathe through the Celtic Resistance.