Quick recap
It's 937. King Edward dies without naming an heir, and Æthelstan, raised by Uhtred, kills his half-brother Ælfweard and becomes king. Swayed by Ingilmundr, his adviser and secret lover, he sets out to unite England by force. Uhtred uncovers Ingilmundr's deceit, is captured, loses Bebbanburg and is banished. Even so, he refuses a plea to kill Æthelstan. Anlaf, a Viking king from Ireland, gathers the northern kings and sacks Bebbanburg. At Brunanburh the Saxons win, but Uhtred is gravely wounded shielding Æthelstan, and Ingilmundr is executed. Uhtred agrees to pledge Northumbria to the king if Edmund is named heir. He has a vision of Valhalla, and the film leaves his fate open.
