Lady Agatha (also referred to as Lady Knocknee) appeared in two Mysterious Theater segments on Sesame Street. She represents the typical grand dame dowager of mystery fiction, summoning the detective to the manor. She first called Sherlock Hemlock and his dog Watson to solve the case of the missing cat (First: Episode 2569). Hemlock, knowing it was under something ending in "at," thought at first that it was Lady Agatha's lovely new hat, and told Watson to find out. Watson ruined the headgear as a result. Lady Agatha's husband, Sir Rodney, and their butler, Woodhouse, also appeared.
Lady Agatha returned solo in "The Case of the Missing Toast" (First: Episode 2943) which also involved the theft of her roast. With rhymes once again as the clue, it turns out the thief was host Vincent Twice, who refuses to return the food to an incensed Lady Agatha.