The Rhinemann Exchange (1977)

David Spaulding is a concert violinist who travels all over the world. Just when the war is about to break he is approached by Army Intelligence to be an agent because his performing all over the world is the perfect cover. Initially he refuses but when his father is killed in an attack by the Germans he accepts and is trained and becomes a seasoned operative. Eventually he is sent to Argentina to broker a deal between a German businessman who offers the plans to German gyroscopes for diamonds. Eventually he learns something odd is going on. And while he tries to accomplish his assignment he tries to sort out the mystery of the exchange.