In the summer of 1956 , Mike Barnett took over Frank Lovejoy 's role until the end of the series in the same year . In the summer of 1956 , Mike Barnett took over the role of Frank Lovejoy until the series ' end that same year . In the summer of 1956 , Frank Lovejoy took over the role of Mike Barnett until the end of the series that same year .