The Once and Future King Book 3, Chapter 11
Lancelot is tortured by his love. He worries that if he gives in to Guenever he will lose his strength, because he believes his purity relates to his powers as a knight. He goes away again, and this time, his quest changes his life. He goes to Corbin to investigate the haunted castle of King Pelles. Once there, he learns that there is a lady kept in boiling water by magic, and feels obliged to help her, since she can only be released by the best knight in the world. Once she is out, he realizes he has done a miracle, as he has always wanted. He learns that the beautiful girl is Elaine, daughter of King Pelles.