When a nun walks into a police station and confesses to murder, the Grayhurst police arrest her for the killing spree that has shocked the city. But Roberta believes Sister Catherine is innocent and sends Jillian and Sam to investigate. Sister Catherine gives them the silent treatment, and all the evidence points to her. Jillian and Sam reluctantly conclude that Roberta must be wrong.

Then Jillian receives a phone call that forces them to reconsider their view of the case. Determined to find out why a nun would confess to murders she didn't commit, Jillian and Sam chase down the new lead with renewed enthusiasm. If Sister Catherine is innocent and they don't uncover the truth, the real killer will get away with murder.