Early one morning Colonel Bohun leaves the home of his mistress, Elizabeth Barnes, the smith's wife who suffers from feelings of guilt over their affair. The smith himself is a fanatic revivalist, who spent the night at a church meeting elsewhere, while Elizabeth is a devout Catholic. To discuss her guilt she has asked a priest visiting the nearby town to come and see her.
The colonel's brother, Reverend Wilfrid Bohun, knows about the colonel's trysts and discovers that some of his parishioners hold the bounder's behaviour against the him. Mrs Deveraux even refuses to have the next church fête in her garden after Colonel Bohun got her maid pregnant. Reverend Bohun visits his brother to confront him with these rumours, but the colonel doesn't seem to be bothered. His wife overhears the painful conversation.
Father Brown arrives in the village and during a visit at the pub he gets up to speed on the local gossip. Waiting to meet Elizabeth Barnes at the Anglican church he strikes up a conversation with Reverend Bohun, who suggests that Father Brown spend the night at the vicarage. In a private confession at her home Elizabeth admits the affair to Father Brown and explains that she finds it difficult to keep the colonel away. She promises to try harder.
That night Colonel Bohun spends a lot of money at the pub, prancing around, wearing an antique samurai helmet. Simeon Barnes informs his wife that he will spend another night away from home at a revivalist meeting. Elizabeth fears that the colonel might turn up at her door again. When that happens, she refuses to let him in, but then he threatens to break down the door.
The next morning Colonel Bohun leaves the Barnes residence and bumps into his brother. Again the colonel seems to show no shame and even takes some time to torment Joe, a mentally retarded villager. Shortly afterwards gravediggers find his corpse in the cemetery; his skull has been smashed by a hammer taken from the smithy. The police presume that Simeon Barnes is responsible, but when he returns home from his church meeting, he can offer an alibi for the time of the murder.
At the pub the police, the local doctor, Reverend Bohun and Father Brown discuss the murder. The colonel's skull was smashed by a heavy blow with medium-sized hammer. Only the smith would have the strength to hit the colonel that hard with a small hammer. Then Reverend Bohun realises that there is someone else with more than average strength: Joe. The police arrest the young man, although he can never be tried as a sane man.
Father Brown is not convinced that Joe was the murderer and goes to have a talk with Simeon Barnes. During the conversation Barnes mentions that the colonel's death was a matter of divine intervention, not a law of nature. This phrase "law of nature" sticks to Father Brown's mind and he develops a new theory. He visits the local church again and climbs the stairs up the steeple. On the roof he finds Reverend Bohun staring at the scenery. He explains his theory: Reverend Bohun was so upset with his brother's treatment of Joe that he took a hammer, ran up the stairs and dropped the hammer from on high, smashing the colonel's head, using the law of gravity. He informs the vicar that he considers this talk a confession and will keep it to himself. As he leaves the village, he sees how the repentant murderer gives himself up to the police.