Riding in search of adventure, Launcelot comes upon a castle where a beautiful lady has been under a curse for five years. He kills a dragon, meets Pellas, son of the Maimed King, and sees the Grail. By a trick, Launcelot is made to lie with Pellas' daughter Elayne, and that night Galahad is conceived. After Launcelot leaves, Sir Bors comes to the castle. He sees visions and hears that though Launcelot is the best of worldly knights, many shall surpass him in spiritual things.
Soon after, Arthur has a feast for his knights, and Lady Elayne comes to it. Launcelot is again tricked into lying with her. Guinevere discovers it and banishes him. Launcelot leaps out a window and runs road from grief. Arthur's knights ride off to hunt for him and have various adventures, including some involving the Grail.
Launcelot meanwhile goes from place to place until the Lady Elayne recognizes him and, through her father, arranges for his healing by the Grail. Believing he is still banished from England, Launcelot asks lodgings from Pellas. He is given a castle, which he names The Joyous Isle, and there he lives under a false name, with Elayne as his servant. In this castle Percival and Ector eventually find him. They return with him to Arthur's hall just in time for the feast of Pentecost, and all the court rejoices at his return.