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The Perfect Sap was based on Not Herbert, a popular stage farce of the period. Ben Lyon stars as Herbert Alden, a wealthy but hopelessly nerdish would-be detective. Herbert gets his chance to prove his deductive skills when a robbery is committed at a costume party. His pursuit of the crooks leads Herbert and his sweetheart Polly Stoddard (Pauline Starke) too a crumbling old mansion, where the villains do their best to convince the couple that the joint is haunted. Featured in the cast were two long-time cronies of W.C. Fields, character actors Sam Hardy and Tammany Young.