Belle Langtry runs a town being taken over by cattle rustlers. She is also a front for the outlaws, who are led by Steve Fraser. Hoppy gets elected sheriff and cleans up the town with help from the Bar 20 boys.
When his horse is conscripted by the government for service in France, Buck Thomas enlists and becomes the orderly to the captain entrusted with the horse.
Dee Halloran rescues Grace Macy from a runaway stage and learns that her grandfather was murdered before he could divulge the whereabouts of a large sum of money he had hidden from thieves. Buck Fitzgerald, the leader of a gang of desperadoes, weasels himself into Grace's confidence and leads her to believe that McKeller, a friend of Dee's, was responsible for the murder. Thinking that Grace knows the location of her grandfather's money, Fitzgerald later tries by force to make her talk.
Brady Hawkes has to run to his son's rescue once again in this continuation of the Gambler stories. Jeremiah is now a young man who has become involved with Butch Cassidy and the Sundance Kid. Brady pursues the gang in order to get Jeremiah out of the gang before he gets in too much trouble with the law.
A tormented and neglected twelve year old finds solace watching classic western movies. When she finds a gun in her late father's loft she stands up to her tormentors, just like her gunslinging heroes.
Passing through a town, a farm boy aspiring to be a bounty hunter rescues a woman who has been tricked into working in its brothel and the two travel towards his father's ranch.
Ken Williams is determined to discover the identity of the mysterious Rattler, who preys upon railroads and transportation companies like that owned by Jane Corwin. The Rattler is especially difficult to catch because of his skill at disguising himself as other people.
Broncho's brother has a quarrel with a greaser. In the fight that follows the greaser is bested, but swears to be avenged. Broncho's brother then goes home and while under the influence of liquor, strikes his mother. Broncho, hearing her screams, rushes into the room, but by this time his brother has departed.
Pecos Martinez rides into Houston looking to settle an old score with gang leader Clane, who is after the money that disappearred from a recent robbery. The hunt for the loot and Pecos' quest become intertwined with the efforts of the locals to stay alive long enough for help to arrive.
Photographed by famed cinematographer Laszlo Kovacs just before embarking on EASY RIDER (1969), Mark of the Gun is a classic western tale of outlaws and the women they love.