Set in Chicago, Illinois, it starred Marvin as Detective Lieutenant Frank Ballinger, a member of "M Squad", a special unit of the Chicago Police, assisting other units in battling organized crime, corruption and violent crimes city-wide. Paul Newlan co-starred as his boss, Captain Grey. Although Marvin had been appearing in feature films since 1951, it was this series that made him a star, and he later went on to an even bigger film career afterward. Nelson Case was the announcer.