A Vampire's Slayer (Nancy Drew: The New Case Files, No. 2)

The Girl Detective has been locked in a large estate with Gregor, the young man whom everyone suspects of being a vampire, only to learn he has a rare disease that won’t allow him to venture outside during the day. While Nancy’s friends George and Bess and boyfriend Ned are locked outside the estate, Nancy and Gregor are trapped inside with a crazed woman who believes Gregor is a real vampire, and that she must destroy him!