"N." is a 54-page novella originally published in the 2008 collection Just After Sunset.
The story concerns the psychiatric treatment of the obsessive-compulsive accountant and amateur photographer who believes that he is responsible for the security of a portal between worlds. The narrative follows the patient's sessions with his doctor culminating with the patient's suicide, which prompts his doctor to investigate the source of his patient's neurosis where the doctor himself falls victim to the same condition.
In the outer circle of a nested narrative, a woman named Sheila writes to her childhood friend Charlie about her brother Johnny, a psychiatrist who recently committed suicide. Sheila suspects it was due to a patient Johnny referred to in his notes only as the eponymous "N."
In the inner circle of the narrative, N. is diagnosed by Dr. John Bonsaint as suffering from obsessive-compulsive disorder and paranoid delusions related to "keeping balance". N. has become convinced that a circle of stones in a field on the outskirts of a nearby town, Ackerman's Field, contains a potential doorway to another reality, where a terrifying monster, repeatedly said to be a "helmet-headed" being named Cthun, is trying to break through. A warning sign of the monster's imminent penetration (best described as a place where the walls between realities are thin, or perhaps breaking down) is when a person viewing the field sees seven stones, when there are in fact eight. N's belief, shared by those who came before him, is that verifying the presence of eight stones when he is in the field, and his obsession with order when he is absent, somehow strengthens the barrier between our world and the one Cthun dwells in, but that it is a perpetual and exhausting struggle. N.'s obsession eventually leads to his death by suicide, despite Johnny's best efforts.
Following a mysterious compulsion, Johnny goes out to the field to see the stones for himself, he begins to suspect that N. might not have been delusional after all when he suffers from the same symptoms as his patient. Most notable are his obsession with numbers: odd numbers are bad, especially prime ones, even ones are safe, especially if they have a lot of factors, and if the sum of their digits is also even. The effect recedes as winter sets in, since the level of danger seems to be synchronized with the solstices (winter is safest, summer is most dangerous). However, as June approaches, Johnny is driven to madness, and finally kills himself.
A newspaper clipping reveals Sheila's fate: after she read her brother's manuscript, she jumped from a bridge near Ackerman's Field and killed herself, in a manner identical to her brother. A copy of an e-mail indicates that Charlie intends to visit the field in Maine.
The story was adapted into a graphic video series by Marvel Entertainment, then as a four-issues series expanding and completing the story (by adding multiple characters, events, and, among other things, revealing "N."'s full name.
"N." was inspired by horror writer Arthur Machen's story The Great God Pan (and not Lovecraft),  and was seemingly named after Machen's story "N.", which shares a few plot similarities with King's story.