This shows the moment in Lancelot's quest for the Holy Grail when he reaches the Chapel where the Grail is kept. As he sleeps he dreams of the angel that guards the grail, she tells him that he will never find the Grail because of his adulterous behavoir with Queen Guinevere.

Interestingly Burne-Jones himself is the model for Sir Lancelot, perhaps he was considering his own affair with sculptress Maria Zambaco.