Madeline and the White Lie - S3.E24

When Lord Cucuface becomes suddenly indisposed, Madeline takes over his mission to impress some American tourists who are potential donors to the girls' school. She tells them that she is a famous tour guide, and finds she must lie more and more as she takes the eccentric couple around the Louvre, until they are all hopelessly lost and she is forced to come clean and tell the truth.