It's supposed to be the story or that is how it is advertised. But personally I think they should have just made up a story and not say it's an Arthur story because it's not accurate. From what I remember Guinevere and Lancelot have an affair once she marries Arthur. The wedding is at the end of the movie and shall we say poor Lancelot isn't going to be having an affair with anyone let alone Guinevere. Really the only things I remember it having in common with the legend is the names and that's it.