Set at a remote Episcopal women's college in Massachusetts in 1968, the novel alternates between the perspectives of two poetry students, Laura Sheridan and Carmilla, as they are drawn into an intense rivalry, a forbidden romance, and the dangerous secrets of their vampire professor.
Laura, a shy, aspiring poet from Mississippi raised by her father after her mother's death, arrives at Saint Perpetua's Women's College as a freshman. Her assigned senior mentor, Maisy Cohen, brings her to the school's traditional Bonfire Night, where seniors crown incoming freshmen with laurels. Carmilla, a dark-haired senior, places a crown on Laura's head, and Laura feels an overwhelming attraction. At her first evening poetry seminar, Laura meets the commanding professor Ms. De Lafontaine, a charismatic woman who memorizes vast amounts of verse and demands the same passion from her students. When Laura reads an original poem aloud, De Lafontaine praises it effusively, provoking visible jealousy in Carmilla. After class, Carmilla approaches Laura and insults her writing as inauthentic, establishing their rivalry.
The narrative shifts to Carmilla's perspective, revealing a hidden dimension to her relationship with De Lafontaine. Carmilla, the daughter of an absent Austrian mother and a father with an alcohol addiction, fled a Swiss finishing school and traveled to America specifically to study under De Lafontaine. Beyond their academic bond, the two share a clandestine arrangement: De Lafontaine is a vampire who regularly feeds on Carmilla's blood while Carmilla recites poetry. Despite this intimacy, De Lafontaine maintains strict boundaries and withholds details about a secret research project. Carmilla breaks into the library at night and discovers that De Lafontaine has been researching the origins of Saint Perpetua's, with a cryptic marginal note suggesting something hidden lies beneath the school.
Laura throws herself into her studies, channeling her attraction to Carmilla into poetry and reflecting on a lifetime of suppressing her dominant sexual desires and her exclusive attraction to women. De Lafontaine invites Laura to a private Friday-night seminar at her apartment, where the only other attendee is Carmilla. The professor pushes both students to produce bolder work and, in one session, devastates Carmilla by attacking her poetic obsession with youth as a symptom of maternal abandonment. Laura follows Carmilla afterward, and the two have their first genuine conversation. Carmilla proposes a formal poetic competition and kisses Laura's wrist to seal it.
Laura's friendship with Elenore Robinson, a fellow writing student, deepens when Elenore casually identifies Laura's attraction to women and expresses complete acceptance. Laura and Carmilla's dynamic shifts further when Laura eavesdrops on an argument between Carmilla and De Lafontaine, then watches through a cracked door as De Lafontaine bites Carmilla's neck and drinks her blood. Caught spying, Laura is summoned to De Lafontaine's office, where Carmilla bluntly confirms that the professor is a vampire. Laura agrees to keep the secret, and De Lafontaine promises to bring her deeper into her world.
At the school's Halloween party, Laura watches Carmilla kiss a townie boy and storms outside in jealous fury. Carmilla follows, and Laura pins her against the building and kisses her. De Lafontaine interrupts and leads them beneath Seward Hall through tunnels to a hidden crypt containing a stone coffin. Inside lies the mummified body of Isis, De Lafontaine's sire, the vampire who created her. Isis abandoned De Lafontaine in the 19th century and later entered a death-sleep beneath the school, where De Lafontaine spent five years searching for her, driven by unresolved love. De Lafontaine cuts Carmilla's palm and pours the blood into Isis's mouth to revive her, but Isis awakens violently, slashing Carmilla's throat and fleeing into the catacombs.
To save Carmilla's life, De Lafontaine drips her own blood into Carmilla's mouth, initiating a vampiric transformation. Carmilla revives but is consumed by ravenous hunger. Only living human blood can sustain a new vampire, and Laura consents to let Carmilla feed from her. De Lafontaine then takes Carmilla to her apartment to recover, shutting Laura out.
Carmilla convalesces under De Lafontaine's possessive care, with De Lafontaine insisting Laura is a passing infatuation. Laura breaks the isolation by sneaking Carmilla out, and the two go skinny-dipping at a nearby lake, where Carmilla feeds from Laura in an intimate, consensual act. A scream interrupts them, and they discover the body of a French exchange student on the quad, her throat torn apart. De Lafontaine initially suspects Carmilla but accepts that Isis is responsible. She reveals that Isis was her lover who abandoned her and entered a death-sleep beneath Saint Perpetua's, and that her years of searching were driven by unresolved love.
Carmilla brings Laura and a group of senior friends to carve their names in the campus bell tower, continuing a secret tradition. On the roof at sunrise, Carmilla expresses vulnerability about her immortality, and the two declare themselves a couple. De Lafontaine takes them to a vampire salon in Boston hosted by Magdalena, a regal, centuries-old vampire whose townhouse serves as a gathering place for vampires and their human companions. Swept up in the hedonistic atmosphere, Laura and Carmilla have sex for the first time. De Lafontaine is visibly cold afterward, separating them on the drive home.
During fall break, Laura visits Elenore's family in San Francisco. Carmilla calls, sounding distressed and begging Laura not to let her go, but De Lafontaine interrupts and the line goes dead. When Laura returns, the two reunite and sneak into De Lafontaine's bedroom while the professor is out. De Lafontaine later discovers blood on her sheets but says nothing.
Defying De Lafontaine's orders, Laura and Carmilla explore the catacombs and find De Lafontaine speaking with the partially regenerated Isis, who demands another victim. De Lafontaine refuses but takes no action against her sire. During a brief Christmas together, the trio experiences genuine warmth, but an unmarked note from Isis arrives threatening that the killings will continue until De Lafontaine returns to her.
The violence escalates. Edith, a ballerina and one of Carmilla's senior friends, is found dead in her locked dormitory, drained of blood. Miranda, a diplomat's daughter who discovers the body, withdraws from school. A third student is found dead on New Year's Day. Carmilla confronts De Lafontaine about her inaction, and the professor breaks down before promising to act. At the campus chapel, De Lafontaine confides in Laura that Isis has demanded Carmilla be surrendered and killed as proof of loyalty. Laura proposes a plan to lure Isis above ground.
The plan unfolds at De Lafontaine's greenhouse. Laura delivers a note to Isis in the catacombs, drawing her to the surface. Isis captures Laura and brings her to the greenhouse, where De Lafontaine holds Carmilla as though preparing to sacrifice her. De Lafontaine presses a sickle to Carmilla's throat, appearing to comply with Isis's demand, but in a swift reversal she slashes Isis's throat and decapitates her. Laura bursts back in to find De Lafontaine cradling Carmilla and weeping. The three bury Isis's body at dawn.
In the spring, De Lafontaine announces she is leaving Saint Perpetua's for a finishing school in France, acknowledging that she modeled her possessiveness on Isis's toxic love. As a parting gift, she gives Carmilla a letter from Magdalena offering both Carmilla and Laura positions in her household for the summer in Spain. Carmilla vows to find De Lafontaine again someday, and De Lafontaine bids them farewell and drives away.
Three weeks later, Laura and Carmilla are living in Magdalena's Boston townhouse, preparing to travel to Europe. Early one morning, Magdalena wakes Laura, cuts her own hand, and fills a teacup with her blood. She explains that Laura may drink and become a vampire, or refuse and remain human. Laura holds the cup to her lips as dawn breaks. The novel ends without revealing her decision, as Laura reflects that she is no longer the frightened girl she once was, seeing herself as a creature of the night regardless of which path she chooses.