The epic poem written by Sir Walter Scott starts from the description of Marmion, the brave and well-known English knight gifted by every virtue that was valued in the age of Henry VIII. Marmion is respected even by the King himself, so he is trusted by Henry VIII to go to the Scottish court with a diplomatic mission. Marmion is expected to negotiate about ceasing the armed raids across the English-Scottish border. After arriving to Scotland Marmion is given a guide, the most thoughtful and peaceful of all the people the Scottish lord who hosted him can provide - a priest who is considered to be a living saint and conducted lots of pilgrimages all over the world.
In the meanwhile we meet the abbess of the monastery nearby who travels with the other nuns to the secluded Cuthbert Isle to conduct a trial. One of the condemned is the young woman named Constance, a nun who broke her vows to be with the man she loved. Another young woman travelling with the abbess is called Clara de Clare. She went to the monastery after her beloved was betrayed and disgraced and Clara herself has to flee to avoid the advances of another man. Clara is a novice, so she hasn’t taken the full vow yet.
On the Cuthbert Isle Constance is awaiting for her fate. The abbess condemns her to be walled alive for her sin. Desperate, Constance confesses to the abbess, telling her the story of her life, love and betrayal. She tells that it was Marmion, the fair knight, known everywhere as embodiment of virtues, who seduced her, made her to commit a major crime against Clara’s fiancé and then purposefully reported her to the abbess.
Marmion promised her to love her forever and marry her if Constance would break her vows and sleep with him. Charmed with the attention of such a handsome and valiant man, Constance agreed. She fled the monastery, dressed up as Marmion’s page and travelled with him for three years. But later she noticed that Marmion got colder to her. He became interested in Clara de Clare, who was as beautiful as Constance, but also was a rich heiress and much more perspective wife than poor nun. The Clara was already engaged with another knight, Sir Ralph de Wilton.
Marmion demanded Constance to help him forge a letter to Wilton that could be used against him. Constance agreed. She didn’t know about Clare yet, so she thought that helping Marmion in destroying another King’s favourite will grant her his attention again. But then, when Wilton was accused of treason and exiled, she understood that all that was for breaking Wilton’s engagement with Clara so that Marmion can marry her.
Constance called him out, but to get rid of her Marmion reported her to the abbess of monastery she was a nun of. Poor Constance was imprisoned and Marmion was finally free to advance at Clara. But the girl preferred to abandon all her previous life if she could not be with the man she truly loved. Despite it was he who caused Constance’s fall from grace and he covered her disguise for three years, Marmion himself wasn’t punished at all, because of his friendship with Henry VIII.
Wilton tried to protect Clara’s honor on the duel but was defeated and seemingly mortally wounded. Marmion’s plan was a complete success. But the last thing that Constance wants to do in her life is to disrupt this plan and to avenge herself. To lift the burden of treason from her soul, the poor woman gives to the abbess the written evidences proving that Wilton isn’t guilty and Marmion falsely accused him. It still doesn’t grant her pardon and Constance is buried alive in the monastery wall, though the abbess promises to pray for her soul.
The abbess tries to find Wilton, but the knight himself has already found a way to re-enter the country. To her great surprise, the quiet and peaceful priest, Marmion’s guide and companion, is Wilton in disguise. He follows Marmion, who wants to return back to the court after successfully completing the King’s task. Marmion plans to ask Henry VIII for a favour: to take Clara de Clare from the monastery and force her to marry him. The King is most likely to please his favourite and satisfy his demand. After learning that, the abbess gives the evidences to Wilton, hoping that it will help to clean his name before the King and accuse Marmion of horrible crimes he committed.
Marmion proceeds with his task. He starts to feel uneasy because of his betrayal but he soothes himself with the thoughts that Constance’s punishment wasn’t too hard. But one day, when he and the “priest” stay in tavern, he hears a young man singing the ballad about the maid, betrayed and killed for her love. The ballad ends with the words that the traitor would never find peace after what he’s done. Suddenly, Marmion hears church bells and the “priest” tells him that the bell tolls for a friend.
This night Marmion is unable to sleep and goes for a ride to calm himself. But suddenly he meets a man with the face of his deceased (as Marmion still thinks) enemy Wilton. Horrified, Marmion manages to escape and return to the inn, but his companion isn’t waiting for him there. He disappeared.
After receiving the documents, Wilton doesn’t need a disguise anymore. He returns to Edinburgh, showing the proofs of his innocence to the Scottish lord who was Marmion’s host. This man, Archibald Douglas, is deeply shocked and disgusted by Marmion’s deeds and immediately restores Wilton in the ranks of the knights, giving him the new horse and armor. Now a knight again Wilton is going to return to Marmion and finish him for good.
In the meanwhile Marmion arrives to the court of Scottish king James and sees there a fair lady named Lady Heron. She is the wife of Archibald Douglas, who was absent in the castle of her husband for a while. The sacred laws of hospitality do not stop Marmion in courting her. But we also learn that Lady Heron is also a mistress of King James himself, so he angrily sends Marmion back to the castle of Tantallon, to Archibald Douglas. While Marmion is heading there, the abbess followed by Clara are captured by Scots on Douglas’ land and are also brought to Tantallon.
But the Battle of Flodden - a real historical event that took place in September 1513 - disrupts everyone’s plans. Neither Douglas nor Wilton don’t have their revenge and Clara evades the hateful marriage again. In this great fight between English and Scottish armies everyone has to fight for his own life.
At the battlefield Marmion sees the “priest” he travelled with, but now he is clad in the armor and sits on the magnificent horse. Finally Marmion recognizes de Wilton and Wilton gives him a bitter and sarcastic monologue accusing Marmion of every his wrong deed. But the duel doesn’t happen again. In the chaos of the battle Marmion is mortally wounded and dies, repenting his sins.
On the contrary, de Wilton fights like a hero, showing extraordinary bravery and wits on the battlefield. He impresses the King so much that he immediately grants him back all his lands and allows him to finally marry his beloved Clara de Clare.