The novel begins with the Editor’s Narrative, in which he tells the story as he knows it before introducing the manuscript. The Laird of Dalcastle married Rabina, a woman whose piety tended toward zealotry and fanaticism. Their strained marriage did result in two sons, but the laird would only recognize the firstborn, George. His mother and the Reverend Robert Wringhim, a strict and passionate spiritual adviser to Rabina, raised the second son, Robert. The dark and bitter Robert grew up nursing an intense hatred of his father and brother, and considered them enemies of himself and God.
Robert began to haunt the steps of George, his brother, and George could not escape his presence and had to withdraw into private. The dispute between the two played out in the town and its environs; at one point George swore he saw his brother’s spectral presence in a hazy cloud atop a mountain. One night after securing victory against his brother in the courts, George celebrated with his friends at a tavern. That night he was mysteriously murdered, and his friend Thomas Drummond was assumed to be the murderer. Drummond fled the country.
Brokenhearted, the laird died not long after. His mistress and adopted mother to George, Miss Arabella Logan, committed herself to finding out what happened to her son. She came into contact with Bell Calvert, a prostitute, who had witnessed the events of that night and could swear that Drummond was not the murderer, but rather that another man who looked like Drummond and Robert had killed him. Bell and Arabella pursued their cause, even coming into contact with Robert and another young man who scared them very much. The evidence they found was able to secure a warrant for Robert but when the authorities went to apprehend him, he was gone. No one knew his fate.
The Editor then presents the memoirs in which Robert tells his story; it is similar to the narrative in the events that it covers, but Robert’s perspective is new.
On his eighteenth birthday, the Reverend announced to Robert that he was now one of the elect. Robert exulted in this, convinced that it was his job to defend the faith and punish sinners. That same day he encountered a young man in the woods named Gil-Martin, whose intellect and spiritual superiority impressed him. He grew dependent on Gil-Martin, and was fascinated by how his friend could change his appearance to resemble those around him. His mother and the Reverend did not like the influence of this young man, but Robert could not live without him.
Gil-Martin began to encourage Robert to get rid of sinners who offended God. He encouraged Robert to kill a local man, Blanchard, whom Gil-Martin did not like and whose preaching (and dislike of Gil-Martin) Robert found blasphemous. Robert was nervous that his infallibility as one of the elect was not absolute, but he finally carried out the murder at Gil-Martin’s urging.
He also explained how he came to murder his brother, to which he was initially opposed. No one knew he did it, and he inherited the laird’s lands. He was initially thrilled to be there, but he became weary of his mother and even of Gil-Martin’s presence. He began to sink into depression, and began to feel estranged from himself. He had constant doubts and began to be told that he had done things he did not remember.
After a night of drinking he woke up and learned six months had passed. He was insensible of what had happened. Terrible things that he had apparently done began to reach his ears; these included murdering his mother and a young woman whose lands he had supposedly taken in order to seduce her. When their bodies were found and people came to apprehend him, he fled.
Poor and miserable, he hoped Gil-Martin would not pursue him—his friend’s presence now unbearable. His life was a horror he could not escape and he felt like his sanity was vanishing. He stayed at various places in his flight but had to leave all of them when people saw that he was persecuted by demons and terrors. During this time he worked on his memoirs. He hoped to publish them but the master of the printing house was horrified by what he saw in the manuscript.
Gil-Martin, now haggard himself, found Robert and told him that there was nothing left for them to do but to kill themselves. Robert had to agree because he was tired of the torment.
The Editor explains how he came to this story. He had seen a letter by the Ettrick Shepherd, James Hogg, in Blackwood’s; in this letter Hogg described the digging up of a suicide’s grave. The Editor wanted to investigate and went to see Hogg. Hogg was dismissive of the endeavor but the Editor traveled to the site and witnessed the digging up of an untouched grave. The corpse was strangely preserved, and had a manuscript with it.
The Editor concludes by saying he does not understand the memoirs, and assumes that even if Robert was involved with George’s death, he thinks the rest is madness. Robert may be a religious fanatic, or crazy. The story may have elements of parable or allegory. Overall, he is quite perplexed by it.