Doc Adams is on his way to tend to Cam Seaton, a man who was injured when he was kicked in the chest by his horse. During the trip, Doc is stopped by a man on horseback. The rider tells Doc to turn back, but Doc refuses. The stranger shoots Doc's horse to prevent him from making it to the injured man, and Doc fatally shoots the man with his shotgun.
Matt Dillon and Chester Goode find Doc's buggy turned over and the dead stranger lying nearby. They ride on to Cam Seaton's house where they find Doc. By the time Doc made it to Seaton, the injured man was beyond help and soon died from his injury.
Doc tells Matt he killed the stranger when the man shot his horse, an act that could be considered murder. Matt does not think Doc's actions warrant arrest. Some of the Dodge City residents disagree.
A stranger soon arrives in Dodge and begins stalking Doc. At Matt's request, Kitty Russell performs some investigative work at the Long Branch Saloon and learns the stranger is named Nate Brandell, and his brother Myles is the man Doc killed. She also learned Myles had a bounty on his head for a crime in Colorado. Nate thinks Doc killed his brother for the bounty money, and Nate wants revenge.
Jack Lord plays both Brandell brothers in this story. This is Lord's only appearance in the series, and it is excellent. Lord can be seen in various roles in several different television shows -- many of those westerns -- in the 1950s and 1960s. He starred as the lead character in the series Stoney Burke during its single season. That series also starred Warren Oates, an occasional Gunsmoke guest and featured Buck Taylor, who would later join the Gunsmoke cast as the character Newly O'Brien, in a few episodes. Lord would later play the iconic character Steve McGarrett in the series Hawaii Five-O.
Longtime Gunsmoke viewers know that Doc killed a few people during the twenty-year history of the series. While Doc's reaction to Myles Brandell shooting his horse is surprising, it is justified under the circumstances. It does not make sense that anyone would think Doc should be prosecuted. (Plus, how did the Dodge City people learn the details of the shooting? Myles Brandell was a stranger to the people of Dodge, and the incident occurred far from the town with no witnesses.)