When Sister Margaret Mary is found murdered in her room, Detective Steve McLean arrives at the school to investigate. As he is discussing the case with the school's Mother Superior, two students, Marjorie Johnson and Sarah Collins, come forward with information that reveals a decades old secret that shocks Mother Superior and leaves Steve with many more questions to answer.
As the investigation proceeds, more secrets are revealed, including a gruesome discovery in the woods behind the school. Each of the secrets brings even more questions and a heightened feeling of danger. When a second murder victim is found, it becomes clear that Marjorie and Sarah are in danger and the killer must be found as quickly as possible.
When Sarah disappears while Steve is at the station looking over new evidence, Marjorie decides that it's up to her to find her friend. Her search leads her to a hidden room in the convent basement, where she finds Sarah with the killer and yet another dead body. While the killer holds the girls captive, the truth and the reasons for the murders are revealed, but the girls will never have a chance to tell anyone else unless help arrives in time.