The poem tells how Lord Marmion, a favourite of Henry VIII of England, lusts for Clara de Clare, a rich woman. He and his mistress, Constance De Beverley, forge a letter implicating Clara's fiancé, Sir Ralph De Wilton, in treason. Constance, a dishonest nun, hopes that her aid will restore her to favour with Marmion. When De Wilton loses the duel he claims in order to defend his honour against Marmion, he is obliged to go into exile. Clara retires to a convent rather than risk Marmion's attentions. Constance's hopes of a reconciliation with Marmion are dashed when he abandons her; she ends up being walled up alive in the Lindisfarne convent for breaking her vows.