Steve McQueen is at his most grumpily enigmatic as he portrays this unorthodox San Francisco police lieutenant. He's none too pleased when the greasy-pole merchant "Sen. Chambers" (Robert Vaughn) has him detailed to guard a shady character who's a potential witness in a case against some hitherto Teflon gangsters. When two gunmen break into his room and gun him down, it becomes imperative that "Bullitt" gets to the bottom of things before his new found pal nails his slats to the mast. With the active support of "Capt. Bennet" (Simon Oakland) who increasingly has to run point for the man, he…