Francesca da Rimini

Paulo is sent by his brother Giovanni to fetch him a bride, and now Francesca waits for him to arrive. This being opera, they immediately fall head over heels in love. Still, she has been promised to the brother, who happens to be older and a powerful and remorseless warrior. This story is drawn from Dante, and the moral seems pretty clear: It's not cool to sleep with your brother's new wife.