Bartram, a lime-burner, and his son Joe watch the kiln on Mount Graylock one night when they hear a slow and solemn laughter resound from the hill below them. A man emerges and introduces himself as Ethan Brand. Bartram remembers that he had heard a story of Ethan Brand – the man who went in search of the Unpardonable Sin eighteen years ago. Brand reveals that he has indeed found the Unpardonable Sin, and it resides in his own heart.
Bartram instructs Joe to tell the townspeople that Ethan Brand has returned, but in his son’s absence, he begins to feel uncomfortable alone with Brand. Bartram remembers the sorts of stories the townspeople used to tell about Brand – they once said that he would converse with the devil through the kiln, together framing the image of a sin that even Heaven’s infinite mercy could not wash away.
Brand explains to Bartram that the Unpardonable Sin is a “sin of an intellect that triumphed over the sense of brotherhood with man and reverence for God, and sacrificed everything to its own mighty claims.” However, he also admits that “Freely, were it to do again, would I incur the guilt. Unshrinkinly I accept the retribution!”
Soon thereafter, three men from town arrive to see Brand. All three men - a stage-agent, a lawyer, and a doctor – honorable earlier in their lives, were now merely drunkards. The three greeted Brand and invited him to drink from a black bottle, in which they claimed he would find something better than the Unpardonable Sin. Brand, who had achieved a “high state of enthusiasm” after his years of solitary meditation, could not bear to be around such “low and vulgar modes of thought”. In their company, he began to worry whether he had indeed found the Unpardonable Sin. He orders them to leave and calls them brutes, offending them greatly. An old man asks Brand whether, during his travels, he had seen the his daughter, a girl who left the village to pursue a life as a traveling performer. Brand remembers that the daughter is the “Esther” of the tale – whom he had made the subject of a psychological experiment, and whose soul he had “annihilated” in the process.
Some youth from the village come up the hillside, at the same time as a German Jew, a showman. The showman allows the youth to look through his diorama, but offends Brand by showing him only a blank canvas. Brand rudely tells the Jew to get into the furnace.
Suddenly, an old dog begins frantically chasing its own tail, causing the crowd to laugh. Brand recognizes a similarity between himself and the dog - both caught in impossible pursuits - and begins to laugh his awful, gloomy laugh. The townspeople are frightened by his laugh and depart, leaving Bartram and Joe alone with Brand once more. Brand tells the other two to go to bed and assures them that he will tend to the kiln.
Alone in the woods, Brand reminisces on his past. He remembers that, tending the kiln years ago, he was a simple and loving man. He had pity for human guilt and woe, and hoped that he might never find the Unpardonable Sin. But, he began to seek intellectual development, which disturbed the balance between mind and heart. He had become a fiend “from the moment that his moral nature had ceased to keep the pace of improvement with his intellect”. His highest effort and the fruit of his life’s labor had produced the Unpardonable Sin.
Brand asked himself “What more have I to seek? My task is done, and well done.” With that, he ran to the top of the kiln, cried his last farewell, and threw himself into the fire.
In the morning, Bartram and Joe find that the mountain seems cheerful and lively, light with sunshine. Bartram opens the kiln, and finds that the marble is burned into perfect, snow-white lime. On top of the lime was a skeleton, also converted into lime. Inside the rib cage, however, rested lime in the shape of a human heart. Bartram asks if Brand’s heart was made of marble – but, after just a moment of consideration, shrugs off the man’s death. Happy that the extra lime from the skeleton will make him richer, Bartram unceremoniously crumbles the relics of Ethan Brand.
Ethan Brand begins as an upright man who cares about others and hopes that all sins are pardonable. Yet, in his intellectual quest, he becomes the most sinful man of all. He becomes obsessed with the question of what the Unpardonable Sin is, and in his eagerness for discovery, manipulates others into committing sin as well; for example, he tricks the “Esther” of the story, the daughter of an old man, with “cold and remorseless purpose” into his “psychological experiment” which “wasted, absorbed, and perhaps annihilated her soul”.
Further, after he returns from his journey, he looks down on all the townspeople, who he now regards as “simple” and “low”. The other men had been his friends; they had come to welcome him, and he had only scorned them. Hawthorne presents these men as drunkards, but also maintains a sympathetic account of their lives. For example, Giles, once a lawyer, had slid from intellectual professions to laborious ones, until he lost a foot and a hand in machinery. Though he was maimed, he was one “whom the world could not trample on, and had no right to scorn...since he had still kept up the courage and spirit of a man, asked nothing in charity, and…fought a stern battle against want and hostile circumstances”. The doctor, whose superior medical skills were not diminished by the alcohol that made him a "brute", also remained a fixture of the community despite his vices.
The German Jew in the story has an ambiguous identity; some believe he is simply an entertainer, while others see him as the Devil. He tells Brand that the Unpardonable Sin is in the diorama, when only an empty canvas is there. This implies that the Unpardonable Sin is nothing, poking fun at Brand’s search.
The Unpardonable Sin can be interpreted in many ways. First, Brand's psychological experimentation on others can clearly be construed as fiendish. Second, his self-imposed alienation leads to a cold view of mankind. He values intellect over compassion and cuts himself off from others. Third, for a sin to be unpardonable, the sinner must be unrepentant. Brand says, "Freely, were it to do again, would I incur the guilt." Lastly, the sin he searches for is within his own heart. This may allude to the Biblical original sin - the sin of knowledge. The quest for the sin becomes the means to its own end; the knowledge itself becomes the sin.
Brand's suicide is ambiguous. If, upon returning home, he comes to understand that the Unpardonable Sin has been with him all this time, his suicide may be the ultimate cry for help. Brand may have sought to be pardoned after all. The scenery in the mountain the next morning aligns with this theme of redemption, as the clouds seemed "almost as if a mortal man might thus ascend into the heavenly regions." Furthermore, the kiln's interior is bright white; Brand's heart rests inside his skeleton in the form of pure lime. This may indicate that a possible purging of sin occurred. In the end, he cries out, “O mankind, whose brotherhood I have cast off, and trampled they great heart behemoth my feet!” Brand recognizes that he may have gained intellectual discovery, but at the great cost of human relationships and strength of heart. His heart, literally turned to stone through years of self-imposed isolation, can now be put to use by Bartram.
An alternative reading of the story explains Brand’s quest as a failure. It is possible that Brand expected his arrival in town to make him into a hero. Instead, he realizes that the townspeople believe he is mad, and recognizes that he never found the Unpardonable Sin. While the other townspeople recognize their failures and join in drinking together, Brand refuses to acknowledge his own failure. Searching for acceptance, he is left with only the fire to turn to. Thus, suicide is his final sin, and one that cannot be atoned for.