Lord Marmion's men idolize him, and tales are told of his duel some time before with Sir Ralph De Wilton- a knight accused of treason- which Marmion won, killing his opponent. Marmion is also a great favourite of the English King, Henry VIII. He is considered such an able and trusted knight that he is sent to Scotland to try to smooth things over a bit, and convince the Scots not to declare war. Lord Marmion needs a guide to lead him through Scotland, and wisely requests a non-military one, such as a priest, in order to appear less warlike to the Scots. He is provided with a Palmer- a religious pilgrim- to guide him. The palmer is a bit strange looking- scruffy and shabby- but is competent, and leads Marmion and his entourage to Edinburgh to meet with the Scottish king, James IV.
Undeterred, Marmion still intends to convince Clara to marry him, but his plans receive a setback when Constance, maddened with jealousy, attempts to poison her. Realizing that having a mistress who is trying to kill his prospective fiancee is just going to be awkward all the way around, he decides that he must rid himself of Constance. He does this by revealing her disguise to the church leadership of the convent. The Catholic Church- especially at that time- didn't have much of a sense of humour about the breaking of holy orders, and Constance is arrested. Right after this, Marmion has to go to Scotland, and he attempts to quiet his conscience by extracting a promise from the Church authorities that Constance will only be incarcerated, not harmed.
Arriving in Edinburgh, the Abbess meets with the Palmer who, it turns out, is De Wilton in disguise. After the duel, though grievously wounded, Sir Ralph was still alive and was nursed back to health by a friend. He then went abroad on pilgrimage for a time, before returning to England to try to clear his name. Clara is overcome with joy that her beloved fiance is still alive, though of course, she can't reveal this in order to keep his secret. Sir Ralph then goes to the Earl of Angus and shows him the documents which clear his name. The Earl, disgusted by Marmion's perfidy, reinstates De Wilton's knighthood and arms him. De Wilton is determined to challenge Marmion, but world events overtake him. Talks break down, war is declared, and preparations are made for the Battle of Flodden Field.
Though England wins the battle, Lord Marmion is mortally wounded during it. Carried back to where Clara is, the dying knight asks her for water. As she gives it to him, Marmion speaks of Constance, and Clara is forced to tell him of her grisly fate. Stricken now with grief and guilt as well as pain, Marmion opines that his secret sins weakened his fighting ability, and he dies. To add insult to injury, after the battle, with all the confusion, large numbers of bodies, and the inevitable looting of the dead, Lord Marmion's body gets confused with that of a servant who was killed nearby. The servant, assumed to be Marmion, is buried with pomp in the Marmion tomb, while the knight himself is buried in a shallow, unmarked grave near where he fell.
In the end, Ralph De Wilton, who distinguishes himself in the battle, is honoured for his bravery and, cleared of all charges, is given back his lands and happily marries Clara.