Mercy (movie, 2000)

Detective Catherine Palmer is on the trail of an elusive serial killer. During her investigation she meets Vickie Kittrie, who belongs to an exclusive club of women who engage in secret sessions of bondage and S&M. Matters become even more complicated when Palmer finds herself attracted to Kittrie, leading to a brief lesbian encounter. Palmer soon learns that each victim belonged to this club of prominent, sexually experimental women. In order to catch the killer, Catherine must trust Vickie to guide her through the dangerous and illicit underground.