King Arthur (Clive Owen) is a Roman cavalry officer, also known as Lucius Artorius Castus, the son of a Roman father and a Celts mother, who commands a unit of Sarmatians Roman auxiliaries Roman cavalry in Roman Britain at the End of Roman rule in Britain in 410. Arthur is loyal to Rome and a devout Christians, but follows the teachings of Pelagius, which many consider heretical. He and his men guard Hadrian's Wall against the Picts, a group of Britons (historical) who are rebels against Roman rule, led by the mysterious Merlin (wizard) (Stephen Dillane).