This question is based on The Once and Future King Book 3, Chapter 7.
Lancelot is captured by four witches, including Morgan le Fay. The witches kept them and even though he is having an affair with Guenever, he must choose one of them to be his mistress, but he did not. Then, a young maiden in a castle offered to help him if he will fight with her father in a coming tournament and he agreed. Lancelot goes to fight Turquine. Lancelot beat him.