It is based on the King Arthur legend, and since the PDF I received with the work is causing the merry devil with my computer, no doubt due to my computer as much as anything, that more or less is all I will say here.
The music itself is high expressionist, high modernist and quite compelling. Certainly it is one of Birtwistle's major offerings. The Royal Opera performance is exceptional.
Beyond that my computer is acting up badly so I must cut this review short. I am very sorry.