Excalibur by Bernard Cornwell is the last in a series of three books, which deal with a fictionalized account of the legend of King Arthur in sixth-century Britain.

The narrative is written by a monk in the Christian monastery of Dinnewrac in the Kingdom of Powys in what is modern Wales. As the tale unfolds, the identity of the monk is revealed as Derfel, once a great warlord and warrior in the court of King Arthur. He describes how Merlin tries unsuccessfully to summon the ancient gods of Britain to come to the aid of the Britons.

Arthur is still suffering the pain and dishonor of having discovered his Guinevere in the arms of Lancelot, who betrayed Arthur by leading a Christian rebellion against him. Guinevere has been imprisoned and Lancelot has fled to his kingdom in the lands of the Belgae, where he plots against Arthur and allies himself with the Saxons in Eastern Britain.

When spring arrives in the south of Britain, the expected invasion of the united Saxon kingdoms takes place, unexpectedly early, before the Saxon feast of Eostre and from the south instead of along the Thames valley. Derfel and his party, including the ex-queen Guinevere are trapped near the city of Aquae Sulis on an ancient, pre-Roman, earth fortified hill called Mynydd Baddon. The Saxons surround Derfel's position believing Arthur is with the group. Eventually, Arthur and the other British warlords arrive to reinforce Derfel and, together, though vastly outnumbered, the British warriors defeat the combined armies of the Saxon kings Cerdic and Aelle.

In the aftermath of the battle of Mynydd Baddon, Arthur relinquishes the throne of Dumnonia and crosses the Severn Sea to take up residence in the kingdom of Siluria; there he passes a peaceful life, practicing the craft of blacksmithing. Derfel often visits Arthur and when he and Arthur believe that King Mordred, the titular ruler of Dumnonia, has died on an expedition to wage war across the sea in France, Derfel returns to Dumnonia to acclaim Arthur's son Gwydre as king.

However, King Mordred is very much alive and kills all the warriors in Dumnonia who are loyal to Arthur. Derfel barely escapes to return to help Arthur embark from Siluria towards Dumnonia. Nimue, Merlin's vicious priestess, attacks the embarking warriors with her army of Bloodshields and, as they escape, executes Merlin on the shore.

In Britain, Arthur's loyal warriors defend him against the army of King Mordred. In the battle of Camlann, at the mouth of the river Exe, Arthur kills King Mordred in single-handed combat but is mortally wounded. He embarks on the ship called Prydwen and sails off into the setting sun, through a silver sea mist, thus fulfilling Merlin's prophesy.