It is a Restoration spectacular,[3] including such supernatural characters as Cupid and Venus plus references to the Germanic gods of the Saxons, Woden, Thor, and Freya.

The tale centres on Arthur's endeavours to recover his fiancée, the blind Cornish Princess Emmeline, who has been abducted by his arch-enemy, the Saxon King Oswald of Kent.

King Arthur is a "dramatick opera" or semi-opera: the principal characters do not sing, except if they are supernatural, pastoral or, in the case of Comus and the popular Your hay it is mow'd, drunk.

For unknown reasons Dryden abandoned his intention to have the whole work set to music and developed the prologue into another opera, Albion and Albanius, a collaboration with the Catalan composer Louis Grabu.

When the Protestant William III overthrew James in the Glorious Revolution in 1688, Dryden refused to renounce his faith and so lost his job as poet laureate to his rival Thomas Shadwell.

With their sources of royal patronage gone, both playwright and composer were looking to make money as freelance professionals and the London stage offered attractive opportunities.

[6] In his preface to the printed edition, Dryden explained he had had to adapt the libretto to the changed political circumstances of 1691: "But not to offend the present Times, nor a Government which has hitherto protected me, I have been oblig'd so much to alter the first Design, and take away so many Beauties from the Writing, that it is now no more what it was formerly..."[7] He also made alterations to suit Purcell's musical needs: "the Numbers of Poetry and Vocal Musick, are sometimes so contrary, that in many places I have been oblig'd to cramp my Verses, and make them rugged to the Reader, that they may be harmonious to the Hearer: Of which I have no Reason to repent me, because these sorts of Entertainments are principally design'd for the Ear and the Eye; and therefore in Reason my Art on this occasion, ought to be subservient to his.

The contemporary writer Roger North was most impressed by Charlotte Butler's singing of Cupid, describing it as "beyond anything I ever heard upon the stage", partly ascribing her success to "the liberty she had of concealing her face, which she could not endure should be so contorted as is necessary to sound well, before her gallants, or at least her envious sex.

"[13] Dryden did not base his libretto on standard versions of Arthurian myth, although he was familiar with such books as Geoffrey of Monmouth's Historia Regum Britanniae.

There are clear parallels between King Arthur and Shakespeare's The Tempest (which Dryden had revised in line with Restoration taste in collaboration with Sir William Davenant in 1667 and which had been turned into a semi-opera with music by Matthew Locke in 1674).

[15] Ellen T. Harris has described the links between the characters: Prospero and Merlin are both good magicians who use an "airy spirit" (Ariel in The Tempest, Philidel in King Arthur) to defeat a potential usurper (Alonzo/Oswald).

[17] In his preface, Dryden explained how he had conducted historical research into Germanic paganism to write the sacrifice scene in the first act: "When I wrote it, seven years ago, I employ'd some reading about it, to inform my self out of Beda, Bochartus, and other Authors, concerning the rites of the Heathen Saxons...".

[18] But Andrew Pinnock believes "practically all the ritual came from a far handier source (which unaccountably Dryden forgot to mention): Aylett Sammes's Britannia Antiqua Illustrata (1676).

Edward J. Dent wrote, "The Frost Scene is one of Purcell's most famous achievements" with "its bold contrasts of style, and the masterly piling up of the music to a climax at the end of the chorus ''Tis love that has warmed us'".

[20] Thomas Gray, commenting on the 1736 production, described it as "excessive fine" and said that the Cold Genius' solo was "the finest song in the play".

[21] This aria ("What power art thou who from below") is accompanied by shivering strings, probably influenced by a scene from Act IV of Jean-Baptiste Lully's opera Isis (1677); but, as Peter Holman writes, Purcell's "daring chromatic harmonies transform the Cold Genius from the picturesque figure of Lully (or Dryden, for that matter) into a genuinely awe-inspiring character — the more so because Cupid's responses are set to such frothy and brilliant music".

[23] The prelude to Act III serves as the basis for the piece Chasing Sheep Is Best Left to Shepherds in Michael Nyman's score for the 1982 movie The Draughtsman's Contract.

(Musical numbers given in bold) Scene 1 The Britons prepare for the battle which will decide who will rule their land: the Christian Arthur or the pagan Saxon Oswald.

He also admits he has lost control of the other spirit, Philidel, "a puleing Sprite" who "Sighs when he should plunge a Soul in Sulphur,/As with Compassion touched of foolish man."

Philidel was supposed to have drawn up the vapours from the marsh and blown them in the face of the Christian soldiers but when he saw the crosses on their banners, he refused to carry out this task.

The Britons sing a song of triumph as the Saxons flee the battlefield: Scene 1 The tender-hearted Philidel pities those soldiers who have lost their lives in the battle.

Philidel tells Merlin that Grimbald is planning to deceive the victorious Britons by leading them to drown in rivers or fall off cliffs.

To pass the time, a "Crew of Kentish Lads and Lasses" entertain them with songs and dances: Oswald and his comrade Guillamar stray from the battlefield, chance upon the pavilion and kidnap Emmeline and Matilda.

Scene 2 Arthur and the Britons are preparing to storm the castle when Oswald comes out and challenges his rival to single combat for the hand of Emmeline and the crown.

Arthur spares his life but tells Oswald he and his Saxons must return to Germany because the Britons "brook no Foreign Power/ To Lord it in a Land, Sacred to Freedom."