Director Mary Harron adapts author Rachel Klein's portentous novel about a teenage boarding-school student who fears that a mysterious newcomer has cast a dark spell over her cherished best friend. In the aftermath of her father's suicide, Rebecca (Sarah Bolger) had a difficult time making an emotional recovery. That all changed when she met Lucy (Sarah Gadon), a fellow student who helped to draw her out of her deep depression. Upon returning for their next semester at Brangwyn College, however, Rebecca and Lucy begin drifting apart following the arrival of Ernessa (Lily Cole), a moody new student who takes an immediate liking to Lucy and accosts her at every opportunity. When Rebecca's teacher Mr. Davies (Scott Speedman) gives his class the assignment of reading Joseph Sheridan Le Fanu's classic gothic novel Carmilla, Rebecca begins to suspect Ernessa of being a vampire intent on consuming Lucy's life force. Those suspicions seem to be confirmed when the students and staff of the school begin dying under mysterious circumstances and Lucy falls gravely ill. But the more Rebecca reaches out to Mr. Davies for help, the more unstable she begins to appear. Left with no other choice in order to save the life of her best friend, Rebecca vows to reveal Ernessa for what she truly is, and in the process makes a disturbing discovery.