This film's title implies that with Billy the Kid and Jesse James dead, Mahoney is the "last of the fast guns." The plot has Mahoney hired by millionaire Reid to find his long lost brother, Franz, who disappeared in Mexico many years ago. If Franz isn't found, his share of the business goes
to Reid's conniving partner, Roland. Roland is determined to get control of the business, but can't do so if Franz is found. Mahoney finds Franz and persuades him to return home, but only after the brother is nearly kidnaped and killed in the process.