Father Brown and Flambeau visit a tomb honoring deceased British General Arthur St. Clare, who is honored throughout England. Father Brown then tells the story of how St. Clare died. During the 19th century, St. Clare and his men fought against General Olivier in Brazil. The conflict ended with St. Clare hanged from a tree and his sword broken in pieces. Father Brown argues that this story does not align with either man’s character and thus cannot be the full story: St. Clare was known as a shrewd military strategist and would never have rushed into a battle that was certain to end in defeat. Meanwhile, General Olivier always treated captured enemies with the utmost decency, sometimes even lavishing gifts on them before setting them free. It is unlikely, Father Brown says, that he would have hanged St. Clare.

Flambeau offers the theory that mental health conditions may have run in St. Clare’s family and that he may have wanted to die by suicide, fearing a coming mental health crisis and wishing to hide it from his family. Since a death by suicide would have been interpreted as evidence of the condition that he wished to hide, he instead rushed into battle hoping to be shot. When he was instead captured, he hanged himself and hoped that others would assume that the enemy had killed him. Father Brown calls this a beautiful, poignant, and honest story but not the true story. Father Brown reveals to Flambeau that St. Clare was a colonizer who—stationed in various colonies of the British Empire—had tortured and enslaved innocent people and stolen vast sums of money. Needing still more money to prevent his crimes from being exposed, St. Clare committed treason, betraying England to its enemies in exchange for gold. When a Northern Irish soldier named Murray discovered the truth about St. Clare, St. Clare murdered him, but the tip of his sword broke off in Murray’s body. Knowing that the broken sword tip would give him away as the murderer, he resolved to hide Murray’s body under a mountain of other bodies. He thus led his men into a doomed charge in order to create the bodies that would hide his crime. After the bloody battle, St. Clare’s men turned on him, and Captain Keith, who was engaged to St. Clare’s daughter, executed St. Clare himself. Father Brown then says that St. Clare is now in the icy lowest level of hell where the traitors reside. They then leave the tomb, and Father Brown encourages Flambeau to forget the story as the rest of the men and England have. Flambeau then slips on ice before seeing a sign that reads “The Sign of the Broken Sword” (150). He expresses annoyance at seeing more of him and his name, and Father Brown says that there are traces of St. Clare and his legacy all over England. He then says that St. Clare’s enemy, Olivier, is honored everywhere else, so he will stay silent about St. Clare.

British society is shaken by the news that the bright and cheerful Scottish comic Sir Aaron Armstrong is dead, seemingly having been murdered. Armstrong, a former Presbyterian in recovery from alcohol addiction, had established a reputation as a good, pleasant man. For this reason, the detective investigating his death with Father Brown asks why someone would have wanted Armstrong dead. Father Brown considers that though he was cheerful, some people are turned off by cheerfulness without humor or wit. They find a bloody knife, a rope, and a pistol at the site, which confuses Father Brown since one person could not have used all these weapons. Merton’s superior, Mr. Gilder, suspects that Armstrong’s Chinese assistant, Magnus, was responsible for his death. However, Magnus says that after the murder, he found Armstrong’s young daughter, Alice, unconscious, having apparently fainted with the bloody knife in her hand. He believes that she murdered Armstrong so that she could be with her lover, of whom Armstrong and Magnus did not approve.

When Father Brown and Merton speak to Alice, she says that she was holding the knife but that she used it to defend her father from his killer. When Father Brown asks who killed Armstrong, Armstrong’s Irish secretary, Patrick Royce, says that he did. He wanted to marry Alice, and Armstrong did not approve, so he got drunk and murdered him, using the rope and pistol. Alice found him and used the knife to try to cut her father free and defend him, but it was too late, and she fainted. Father Brown does not believe him, however, finding his story illogical due to the number of weapons supposedly used. Alice tries to back up Royce’s story to Father Brown, but he stands by his assertion that Royce is lying. Father Brown then explains that Armstrong’s death was a suicide. Armstrong, having built his life and reputation on perpetual cheerfulness, was in fact deeply depressed. The expectation of constant cheer had caused his depression and triggered a relapse of his alcohol addiction. Armstrong then tried to take his own life, scattering possible implements of death—the knife, the gun, and the rope—around the room in a frenzy. Patrick found him and tried to stop him, first by firing the pistol to startle him back to his senses and then, when that failed, by tying him up with the rope to prevent him from jumping off the balcony. At that moment, Alice walked in and, misunderstanding the situation, used the knife to free her father, cutting Royce’s knuckles in the process. As soon as he was free, Armstrong ended his life by jumping from the balcony. Father Brown says that he understands that Alice and Royce blame themselves, but he believes that they have a chance to be happy together. He then leaves as the inquiry into the case begins.