You are mine, you shall be mine, you and I are one for ever.

When a mysterious carriage crashes outside their castle home in Styria, Austria, Laura and her father agree to take in its injured passenger, a young woman named Carmilla. Delighted to have some company of her own age, Laura is instantly drawn to Carmilla. But as their friendship grows, Carmilla's countenance changes and she becomes increasingly secretive and volatile. As Carmilla's moods shift and change, Laura starts to become ill, experiencing fiendish nightmares, her health deteriorating night after night. It is not until she and her father, increasingly concerned for Laura's well-being, set out on a trip to discover more about the mysterious Carmilla that the terrifying truth reveals itself.