Season 1: AD43. As the Roman Army prepare to destroy the Celtic land of Britannia, rival warrior women Kerra (Kelly Reilly) and Antedia (Zöe Wanamaker) find themselves working together to fight off the monstrous General Aulus Plautius (David Morrissey), the leader of the Roman invasion.

Season 2: Two years after invading, General Aulus is Romanising cooperative tribes and crushing resistance with the help of Queen Amena (Annabel Scholey), but does he have another agenda? The only form of hope for the Celts and Druids is Cait (Eleanor Worthington-Cox), a young girl who is being trained by outcast Divis (Nikolaj Lie Kaas) to fulfil a prophecy that would save Britannia from the Romans.