The Reverend Wilfred Bohun is pleading with his wanton brother, the colonel, to leave the blacksmith’s wife alone while the blacksmith is away. Angry at his brother’s unrepentent lust, the curate warns him that God may strike him dead and runs into the old Gothic church to pray. Half an hour later, the village cobbler enters the church to tell the curate that his brother is dead. They run out to find Colonel Bohun’s corpse stretched out in the courtyard of the smithy, his head smashed in by a small hammer. The local police inspector and doctor are already trying to reconstruct the crime. At first it seems obvious: The blow was so powerful, smashing even the colonel’s metal helmet, that only the smith could have delivered it—and the smith, indignantly aware of the colonel’s trifling with his wife, had ample motive. The smith, however, is soon cleared by unimpeachable witnesses who place him in the next town at the time of the crime.
The hammer’s smallness moves the doctor to guess that the smith’s wife killed the colonel. This theory is discarded, however, because the fatal blow was too powerful for her to have dealt. The curate suggests that the village idiot, Mad Joe, might be capable of such a blow. He recalls seeing Mad Joe, the smith’s nephew, praying in the chapel just before the murder, and seeing his brother, the colonel, mercilessly teasing the poor soul as he left the chapel.
The only one who does not seem content with the curate’s solution to the mystery is Father Brown, who suddenly becomes noticeable in the crowd. When he is alone with Reverend Bohun in the spire of the church, he offers his own solution to the mystery: The murderer is Bohun himself, and the mysterious force that crushed his brother’s metal helmet with such a small hammer is a natural one: gravity. Having picked up a hammer while pleading with the colonel at the smithy, Bohun threw it from the top of the belfry onto his brother’s head, the acceleration giving the tiny tool tremendous force. Father Brown swears that he will not reveal Bohun’s secret but urges him to give himself up, which the curate does immediately.