I Love Lucy (1951)

S3E9 Too Many Crooks

Lucy secretly enters the Mertzes' apartment to borrow one of Fred's suits, hoping to have a custom tweed suit made for his upcoming birthday. Meanwhile, Ethel becomes suspicious and mistakenly believes that Lucy is the infamous local burglar known as "Madame X." Determined to uncover the truth, she begins to discreetly observe her friend to gather evidence.