The plot of Book 3 is familiar to most fans of Arthurian literature. Lancelot travels from Brittany to England, with the dream of helping Arthur establish a kingdom based on justice and chivalry. He quickly becomes Arthur’s greatest knight, and has many adventures. He also falls in love with Arthur’s wife, Guinevere.
The Arthurian love triangle is widely considered to be one of the most poignant and greatest love stories ever written. It works so well because all three characters love each other, so none of them can ever be happy at the expense of another.
At this point in the story, Lancelot and Guinevere do not act on their love. Theirs is an affair of the heart.