John Wayne plays a lawman who has to deal with the problems of fatherhood in a big way in Cahill: United States Marshall. Wayne is J.D. Cahill, whose singular desire to track down law breakers strains his relationship with his two teenage sons --17-year-old Danny (Gary Grimes) and 12-year-old Billy Joe (Clay O'Brien). The film begins as Cahill is hot on the trail of a gang of outlaws. After the big showdown, he returns to town to discover that the local bank has been robbed. The sheriff and the deputy have been killed, and four bank robbers are imprisoned in the jail. He is stunned when he finds out that one of the robbers in jail is Cahill's son Danny. It seems that during Cahill's absence from home, his two sons have been enticed into a criminal life by nefarious outlaw Abe Fraser (George Kennedy).