The subject is taken from Sir Thomas Malory's Morte D'Arthur, a fifteen-century prose epic. Four legendary queens contend for the love of Sir Launcelot; one of them, Morgan le Fay, casts a spell on him as he sleeps. The swan is the knight's dream image of his true love, Guenevere.

Jones shows Launcelot with a German helmet, referring to the first world war was which dominates much of his work. The landscape is based on that near Capel-y-ffin in the Black Mountains. Jones was part of the Catholic artists' colony there, formed by the artist Eric Gill in 1924.