The queen decides to go on a May Day ride with her ten knights — unarmed except for swords — along with their ladies and their servants. Since Launcelot is not going to be among them, Sir Melliagaunce, who has long lusted after the queen, decides to attack her party and ravish her. Guinevere's knights fight but are overmatched, and to save their lives the queen agrees to go with Melliagaunce. She gets a message off to Launcelot, and he comes after her. When Melliagaunce's archers kill his horse, he seizes a woodcart and comes in that. Melliagaunce pleads through the queen for mercy and, though still furious at the murder of his stallion, Launcelot pardons Melliagaunce.
That night Launcelot goes to the queen's room, tears an iron grill from her window, cutting his hand, and at her request lies with her. Melliagaunce sees the blood on the bed in the morning and accuses her of faithlessness to Arthur. To save Guinevere from execution at the stake, Launcelot says he will be her champion and sets a day for trial by battle.
Melliagaunce treacherously drops Launcelot through a trapdoor into a dungeon, but Launcelot escapes through the love of his lady jailor. When he has beaten Melliagaunce, he offers to end the fight with one hand lashed to his body and his left side exposed, for he would rather fight thus than grant Melliagaunce mercy. Melliagaunce stupidly accepts the offer and dies.