Madame X (Alice Wills) is the main villainess from "Too Many Crooks," episode 3.08 of I Love Lucy (airdate November 30, 1953). She is a notorious burglar who breaks into homes and steals valuables, all while dressed in men's clothing. The villainess' choice of attire is mainly to throw suspicion off her; making police believe that a man is committing the thefts.
After the Ricardos and Mertzes read about Madame X, numerous coincidences led to Lucy and Ethel accusing each other of being Madame X. Lucy was accused after Mrs. Trumbull informed Ethel that she spotted Lucy with a man's suit, which was revealed to be Fred's (though Lucy and Ricky were planning to buy Fred a new suit for his birthday). During the Mertzes' attempt to prove (or disprove) the claim regarding Lucy, Ethel ended up wearing Fred's coat and hat due to feeling chilly, only for Lucy to spot the silhouette and believe that Madame X was outside. After Lucy spotted Ethel inside the building in Fred's clothing, she was left to believe that Ethel turned heel and became the villainous Madame X.
In the episode's climax, Lucy hid in waiting inside her apartment, as she was looking to catch Ethel as Madame X. However, the real Madame X appeared and committed her latest theft, stealing the Ricardos' valuables. Believing that it was Ethel in the guise, Lucy angrily approached Madame X, who later pointed a gun at Lucy. Lucy swatted the gun away and chased the villainess around the living room and subdued her. However, when Ethel entered Lucy's apartment, Lucy realized that she had the real Madame X pinned down, which caused both women to panic while Madame X escaped.
Madame X was subdued by Ricky, who was also under the impression that it was Ethel. After seeing Ethel with Lucy, Ricky panicked when he realized that he had the real Madame X. Later on, Fred entered the apartment with police officers, who captured and arrested the evil Madame X.