I remember this show very well because it was the first time I ever saw Jack Elam as a hero instead of the steady stream of villains he played up to that time. If I remember correctly, the four guys had been outlaws at one time, maybe went to prison or not but, were persuaded to become lawmen, U.S. Marshalls, I believe. And out of the cast, only Elam and Chad Everett became household names.
