The story is presented by Le Fanu as part of the casebook of Dr Hesselius, whose departures from medical orthodoxy rank him as the first occult doctor in literature.[2] The story is narrated by Laura, one of the two main protagonists of the tale.
Laura begins her tale by relating her childhood in a "picturesque and solitary" castle in the midst of an extensive forest in Styria where she lives with her father, a wealthy English widower, retired from the Austrian Service. When she is six years old, Laura has a vision of a beautiful visitor in her bedchamber. She later claims to have been bitten on the chest, although no wounds are found on her.
Twelve years later, Laura and her father are admiring the sunset in front of the castle when her father tells her of a letter he received earlier from his friend General Spielsdorf. The General was supposed to bring his niece, Bertha Rheinfeldt, to visit the two, but the niece suddenly died under mysterious circumstances. The General ambiguously concludes that he will discuss the circumstances in detail when they meet later.
Laura is saddened by the loss of a potential friend, and longs for a companion. A carriage accident outside Laura's home unexpectedly brings a girl of Laura's age into the family's care. Her name is Carmilla. Both girls instantly recognize the other from the 'dream' they both had when they were young.
Carmilla appears injured after her carriage accident, but her mysterious mother informs Laura's father that her journey is urgent and cannot be delayed. She arranges to leave her daughter with Laura and her father until she can return in three months. Before she leaves she sternly notes that her daughter will not disclose any information whatsoever about her family, past, or herself and that Carmilla is of sound mind. Laura comments that this information seems needless to say, and her father laughs it off.
Carmilla and Laura grow to be very close friends, but occasionally Carmilla's mood abruptly changes. She sometimes makes unsettling romantic advances towards Laura. Carmilla refuses to tell anything about herself or her background, despite questioning from Laura. Her secrecy isn't the only mysterious thing about her. Carmilla sleeps much of the day, and seems to sleepwalk at night. When a funeral procession passes by the two girls and Laura begins singing a hymn, Carmilla bursts out in rage and scolds Laura for singing a Christian song. When a shipment of family heirloom restored portraits arrives at the castle, Laura finds one of her ancestor, "Mircalla, Countess Karnstein", dated 1698. The portrait resembles Carmilla exactly, down to the mole on her neck.
During Carmilla's stay, Laura has nightmares of a fiendish cat-like beast entering her room at night and biting her on the chest. The beast then takes the form of a female figure and disappears through the door without opening it. Laura's health declines and her father has a doctor examine her. He speaks privately with her father and only asks that Laura never be left unattended.
Her father then sets out with Laura in a carriage for the ruined village of Karnstein. They leave a message behind asking Carmilla and one of the governesses entreated to follow after once the perpetually late-sleeping Carmilla wakes up. En route to Karnstein, Laura and her father encounter General Spielsdorf. He tells them his own ghastly story.
Spielsdorf and his niece had met a young woman named Millarca and her enigmatic mother at a costume ball. The General's niece was immediately taken with Millarca. The mother convinced the General that she was an old friend of his and asked that Millarca be allowed to stay with them for three weeks while she attended to a secret matter of great importance.
The General's niece fell mysteriously ill and suffered exactly the same symptoms as Laura. After consulting with a priestly doctor who he had specially ordered, the General came to the realization that his niece was being visited by a vampire. He hid in a closet with a sword and waited until seeing a fiendish cat-like creature stalk around his niece's bedroom and bite her on the neck. He then leapt from his hiding place and attacked the beast, which took the form of Millarca. She fled through the locked door, unharmed. The General's niece died immediately afterward.
When they arrive at Karnstein the General asks a nearby woodsman where he can find the tomb of Mircalla Karnstein. The woodsman relates that the tomb was relocated long ago, by the hero who vanquished the vampires that haunted the region.
While the General and Laura are left alone in the ruined chapel, Carmilla appears. The General and Carmilla both fly into a rage upon seeing each other and the General attacks her with an axe. Carmilla flees and the General explains to Laura that Carmilla is also Millarca, both anagrams for the original name of the vampire Countess Mircalla Karnstein.The party is then joined by Baron Vordenburg, the descendant of the hero who rid the area of vampires long ago. Vordenburg is an authority on vampires and has discovered that his ancestor was romantically involved with the Countess Karnstein, before she died and became one of the undead. Using his forefather's notes he locates the hidden tomb of Carmilla. An Imperial Commission is then summoned who exhume and destroy the body of the vampire on behalf of the ruling Habsburg Monarchy, within whose domains Styria is situated.
Afterwards, Laura's father takes her on a year-long vacation to recover from the trauma and regain her health, but she remains haunted by memories, about which she has ambiguous feelings: she acknowledges that the "girl" Carmilla was actually a monstrous predator from whom she barely escaped with her life and soul intact, but nevertheless, she misses her.
Italiano
Laura, una ricca fanciulla di origini inglesi vive col padre in un isolato castello della Stiria austriaca. Da molto tempo aspetta l’estate che dovrebbe trascorrere con la nipote del generale Spieldorf, ma purtroppo la giovane, sua coetanea, muore.In una notte di luna piena, mentre Laura è in giardino con le governanti e il padre, una carrozza esce di strada proprio davanti al suo castello. Le viandanti sono un’elegante signora e sua figlia che per il colpo è svenuta. Dopo i primi soccorsi la signora racconta di avere delle faccende urgenti da sbrigare, così il padre di Laura si offre di ospitarne la figlia sino a quando non tornerà. La signora accetta la cortesia e confida al gentile signore che sua figlia è cagionevole di salute e soggetta a crisi di nervi. Così la misteriosa donna riparte in tutta furia, lasciando lì la giovane. La fanciulla in questione, di nome Carmilla, dall’incarnato splendente e con lunghissimi capelli scuri dai riflessi dorati, è molto bella e ha più o meno l’età di Laura, che rimane estasiata dalla visita, vista la prematura morte della cara nipote del generale. Carmilla e Laura stringono subito un forte legame, Laura adora la nuova compagna che le dimostra molto affetto e tenerezze forse inusitate, ma nonostante ciò non può non notare alcune strane abitudini dell’amica: si desta molto tardi, odia i canti religiosi e assomiglia in modo incredibile ad un dipinto di Mircalla, contessa di Karnstein, che duecento anni prima fu la signora di quella terra.
Nel frattempo si avvicendano vari eventi strani: la morte di numerose fanciulle nel villaggio, una terribile visione onirica che provoca a Laura una strana malattia, le scomparse notturne di Carmilla e infine l’arrivo del generale Spieldorf. Per sbrigare una questione urgente, Laura, suo padre e il generale si recano alle rovine di Karnstein e durante il viaggio il generale racconta gli eventi concernenti la morte della nipote: durante un ballo lui e la nipotina conobbero una bella dama e sua figlia Millarca che fu ospite a casa loro, ma dopo poco tempo la fanciulla si rivelò un vampiro mentre sua nipote cominciò ad avere strani sogni, si ammalò e morì. Nel frattempo arrivano a Karnstein, dove la bellissima residenza cade in pezzi; il generale allora comincia a narrare la storia di quel luogo e dei suoi signori, i Karnstein, che furono creature spietate e sanguinarie. Mentre narra questi eventi giunge la giovane Carmilla e riconoscendo in lei la terribile Millarca subito cerca di colpirla, ma Carmilla lo blocca senza sforzo con la sua delicata mano e poi svanisce. Il generale non ha più dubbi e in attesa di una certa persona va al castello con Laura e suo padre.
Arriva dunque un esperto di vampiri, il barone Vonderbug, e insieme la compagnia - tranne Laura - torna alle rovine di Karnstein dove tra i rovi viene ritrovata la tomba della contessa Mircalla. Quando viene aperta, al suo interno non vi si trova uno scheletro, bensì la bellissima Carmilla integra nella sua immortale bellezza intinta nel sangue. Così la creatura viene giustiziata, le si conficca un paletto nel cuore, viene decapitata e le sue spoglie sono bruciate. Laura non assiste alla fine atroce dell’amata amica, ma conosce i raccapriccianti particolari grazie al resoconto della Commissione Imperiale.
Il barone Vonderbug infine svela gli arcani di questo caso a partire dal nome della vampira: Carmilla e Millarca sono semplicemente anagrammi del nome della contessa Mircalla. Duecento anni prima un suo antenato della Moravia arrivò in quella zona della Stiria dove conobbe e s’innamorò della giovane Contessa Mircalla, purtroppo la cagionevole fanciulla morì presto. Il barone aggiunge anche che i vampiri originali prendono vita quando giovani persone muoiono in situazioni drammatiche attaccandosi al mondo terreno e questo temeva fosse il caso di Mircalla. Il suo antenato conosceva bene come venivano uccisi i vampiri e per evitare che la sua amata subisse lo stesso trattamento ne nascose la tomba. Soltanto nella vecchiaia ripensò a ciò che aveva fatto e così scrisse un resoconto sul caso di Mircalla e su come ritrovare la sua tomba. Così dopo duecento anni, in cui Mircalla, Millarca o Carmilla aveva seminato morte per restare eternamente giovane, il demonio viene ucciso. Laura tuttavia non riuscirà mai a dimenticare la cara amica.