Brought up in poverty, Hyacinth has nevertheless developed aesthetic tastes that heighten his awareness of the sordid misery around him. He is drawn into the secret world of revolutionary politics and, in a moment of fervour, makes a vow that he will assassinate a major political figure. Soon after this he meets the beautiful Princess Casamassima. Captivated by her world of wealth and nobility, art and beauty, Hyacinth loses faith in radicalism, 'the beastly cause.' But tormented by his belief in honour, he must face an agonizing, and ultimately tragic, dilemma.
