The novella is set in 19th-century Styria, which is located in Austria. It is narrated by a young woman named Laura, who tells her story to Doctor Hesselius, whose papers are being organized by an aide.
Laura lives with her father (a widower) in a castle deep in the forest of Styria. It is an isolated but beautiful and serene place. Besides the servants, the only inhabitants of the castle are Madame Perrodon, a governess, and Mademoiselle De Lafontaine, a finishing governess.
Laura narrates that her first distinct memory of her life is from when she was six years old. She awoke in the middle of the night without any nurse or other attendant. She was not afraid until she saw a lovely young woman at the foot of her bed. The woman came and laid down with her and they fell asleep. Laura then awoke to the sensation of two piercing needles below her throat, and the woman fled to under her bed. When her nurse and others came, they felt the spot next to the child and realized it was warm, but Laura’s father dismissed it all as a bad dream. Laura remained afraid of the night for a long time afterward.
At present, Laura and her father are expecting General Spielsdorf, a neighbor, and his niece, a young woman Laura’s age with whom she is excited to spend time since she is often lonely. A letter arrives bearing bad news, though—General Spielsdorf says his niece is dead, and rambles incoherently about seeking vengeance. Laura is very disappointed.
That same day though, Laura and her father espy a carriage coming toward them at breakneck speed, and it crashes near their castle. Everyone seems to be okay, including an older, elegant lady and her young daughter, who is unconscious but alive. The old woman says she has urgent business and wonders if she can leave her daughter here. Laura is thrilled because the young woman is beautiful and charming, and she has been cruelly deprived of the General’s niece. Laura’s father agrees. The old woman pulls him aside and says that her daughter, Carmilla, will say nothing of herself and where she is from; it is a matter of importance.
Carmilla is brought inside and a doctor visits and proclaims that she is going to be fine. When Laura meets her for the first time, she is struck anew by Carmilla’s loveliness as well as by the fact that Carmilla was the young woman from her encounter when she was six. Carmilla tells her quickly that she too had a similar encounter when she was a young girl, and in it Laura was the older girl; she concludes that they were meant to be friends and Laura agrees. Carmilla is open, warm, and effusive with Laura, who wonders at this level of confidence. She feels a tiny degree of repulsion but the attraction overwhelms her.
The two become fast friends and are enraptured with each other. Carmilla has a few odd habits, such as locking her door from the inside at night, not coming down until one in the afternoon, and displaying an intense lassitude. She also has moments when she is utterly consumed by a passion for Laura and kisses, grasps, and tells her they will be one. Laura finds this odd but cannot help but return Carmilla’s affection. Laura wishes Carmilla would tell her more about where she came from, but Carmilla firmly but kindly rebuffs her questioning. As for religion, Laura learns Carmilla was baptized but she never takes prayers with them, and one day expresses disdain for a retinue of peasants singing a hymn. She does seem superstitious though, purchasing a charm from a traveling hunchback against the “oupire” (vampire) said to be in the region and urging Laura to do so as well.
Laura and her father come to realize that Carmilla is probably descended from the same line as Laura’s mother—the Karnsteins, an ancient and noble family in the region whose name and title have long since died out. This is brought to light when a portrait of Mircalla, Countess of Karnstein, is brought back to the castle after having been away for cleaning, and its reveal shows Mircalla as a double for Carmilla in all respects.
A few troubling incidents in the region make their way to the household. It seems that the General’s daughter wasn’t the only young woman to die of an odd ailment, and that a few others have as well.
Laura has another odd experience one night, seeing a black cat-like creature on her bed advancing toward her. It pierces her throat and when she awakes, screaming, she sees a female figure at the foot of her bed, extremely still and ominous. It vanishes, but the terror Laura feels lingers into the next day. She does not tell her father, worried he will think she has the mysterious affliction, but she does tell her governesses. Madame Perrodon mentions that Martin, a groundskeeper, saw a young woman walking in the lime tree avenue and thinks the place is haunted.
Laura tells Carmilla what happened and Carmilla ventures that the same thing happened to her. She says Laura ought to sleep with the charm. Laura does, and has no more troubling encounters, but begins to feel a strong sense of languor. Carmilla is more devoted than ever as Laura begins to sink into her melancholy. She has odd dreams and thinks she speaks to people whom she cannot remember. While she has no bodily pain, she does waste away in her vigor and motivation.
One night, Laura has a dream that her mother is warning her of an assassin, and she sees Carmilla in her room. Carmilla vanishes, and Laura rushes out to see if the real Carmilla is okay. She will not open her door and a servant is called to force it open. Carmilla is missing and the whole household is frightened for her. Finally she reappears, and Laura’s father tries to calm them all down by saying Carmilla was simply walking in her sleep.
Laura’s father calls a doctor to see her, as she is not improving. The doctor questions her at length, and then orders her not to be alone for even one second. Laura and Madame Perrodon are curious as to what the doctor said to Laura’s father about her condition.
Laura’s father receives a letter from General Spielsdorf saying he will be there soon. At the moment though, he would like to travel over to the ruined village with the Karnstein tombs. Madame and Laura will accompany him now, and Mademoiselle and Carmilla can meet up with them later when Carmilla awakes.
Along the road to Karnstein, they encounter the General. He sends his horse ahead and joins them in their carriage. He is much altered, looking older but filled with a passion and an anger. He speaks of how he must have vengeance against the fiend that killed his niece. Laura’s father asks what is going on and the General says they will not believe him, but he will tell his story anyway. He clarifies that they are going to a ruined chapel and the tombs of the ancient family in this village, and Laura’s father assents. The General grimly says he will be unearthing some of the Karnsteins and getting rid of the monsters.
He begins his story, which is very similar to what has been going on with Laura. At a fancy fete hosted by the Grand Duke, the General and his niece meet a beautiful young woman, Millarca, and her older mother. The mother claims she knows the General from the past and has memories and stories to prove it, though he has no memory of her. The young women become fast friends. The older woman is approached by a man with a pale face, and after they talk, the old woman tells the General she must go away on urgent business and wonders if her daughter might stay with them. The General is unsure, but the niece and Millarca beg him to let Millarca stay. He assents, and after getting to know the girl a bit better, grows fond of her charm and wit. Millarca comes to their home (after vanishing for a bit and then turning up unexpectedly). Almost immediately, his niece’s health begins to deteriorate. She complains of dreams and nightly sensations such as being pierced and strangled.
As the General tells this story, they arrive at Karnstein. It is a gloomy, silent place but the General is riled up about it. He calls over a nearby woodman and asks how it came to be deserted, and the woodman tells him about vampires ravaging the place. Even when a Moravian nobleman destroyed one, the place was still too beleaguered to endure.
The General continues telling his own story to Laura and her father, explaining that he called in two doctors who argued over what was going on with his niece. One of them, Dr. Gratz, told him privately that this was no natural illness and gave him a letter with instructions. He also told him to read it in front of the priest. The General read it on his own, not finding the priest, and was startled to read that the doctor believed it was a vampire attacking his niece. That evening, upon instructions, he waited and saw a creature near his niece while she was sleeping. He yelled and scared it off, but not before he recognized Millarca. The next morning, his niece died.
Laura grows gloomy hearing this tale, but brightens when she sees Carmilla drawing near. When the General espies Carmilla though, he jumps up as if to attack her. Carmilla looks fiendish and brutish, staving off the General’s attack and vanishing. He says soberly that it is Mircalla, Countess Karnstein, and the one who destroyed his niece—Carmilla, Millarca, and Mircalla are one and the same.
A strange man joins them. It is the Baron Vordenburg, a man who has devoted his life to pursuing vampires. He is a lanky man with gold spectacles and dressed all in black. With the help of the General, Laura’s father, a small book, and a map, he figures out where Mircalla’s tomb is located. The General says he will bring the law and go about this the appropriate way.
Laura’s father procures the priest and that night she is well-guarded. She has no idea where Carmilla is and is quite concerned.
She learns later about the legend of the vampire, and comes to realize it is what haunted her. The next day, her father, the General, and men of the law and medicine gather at Mircalla’s tomb and open it. Mircalla, or Carmilla/Millarca, is there, rosy and warm as if she were alive. She is resting in seven inches of blood. They stake her, decapitate her, and burn her, and no vampire haunts the region anymore.
Laura concludes her narrative with an overview of the Baron’s visit for a few weeks at her home, during which he shares with her father numerous stories and accounts of vampires. He even tells them that the Moravian nobleman who killed the vampire in Karnstein was said to be a lover of Mircalla. When she committed suicide, he feared she would become a vampire and so he moved her remains and her tomb. When he was older, he realized this did not, and could not, work, so he wrote about the location of the tomb and a way to destroy her, but he died before getting to do so. This is how the Baron, a descendent, came to have the information regarding the tomb’s location.
Laura and her father go to Italy for a year to forget what happened, but sometimes Laura still hears light footfalls as if Carmilla is outside her door.
