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Movies like Martha (1974)

Martha overhears Bernard and John talking about blackmail. Jacob has come up with a plan that includes a mysterious key. Brothers Bobby and Arnold, eternal rivals, both fancy Martha, but also have a soft spot for Leonie. The latter gets tasked to learn more about Jacob's plan. A dead man is discovered lying in Bernard's office late at night by somebody else searching the premises. When the main conspirators all gather at the Zevenhuis villa, John declines to cooperate...