She hired him to watch her.

Tom Beckwith has just quit his job as a police officer in order to study psychology. But his professor, Lauren Porter, is more interested in his criminology abilities than in his progress in psychology. Because of traumatic circumstances during her childhood she is suffering from multiple personality disorder, and it looks like her other ego may have killed someone. So Tom tries to help her by observing her, combining his professional abilities as an ex-cop and his recently gained knowledge of psychology.