The Secret of My Success (1965)

"How three beautiful girls love for fun - and murder for profit."

Arthur Tate is a dull-minded, low-ranking police officer in a small English town. However, with help from his kindly, perceptive mother, Arthur solves a murder committed by a local seamstress, takes down a baroness who has been breeding terrifyingly large spiders, stops a revolutionary attempting to overthrow a Third World government, and might just manage to secure a big promotion.