John Dryden wrote the play King Arthur, and Henry Purcell wrote 90 minutes of orchestral and vocal music for it, turning it into a masque. Mark Morris, whose soul seems to have traveled across time from the Baroque, tossed John Dryden's play out the window, and has presented only Purcell's music as a ballet with singing that he calls King Arthur.
The Morris project is certainly not a masque, or an opera, or a musical, nor does it have a story. Um, nor does it now involve the character King Arthur in any way. The Times of London called it an "entertainment." That's precisely the term. Whatever it is, it's delightful; just don't expect any element of 17th-century theater.