King Arthur , subtitled "The British Worthy", is an opera in five acts, with a libretto by John Dryden and music by Henry Purcell. The opera was first performed at the Queen's Theatre, Dorset Garden, London, in late May or early June of 1691. The plot is based on the battles between King Arthur's Britons and the Saxons, rather than the legends of Camelot (although Merlin does make an appearance).