Impulse (1990)

Regularly working alone undercover with a vice squad boss she dislikes as much as he dislikes her, Lottie Mason is living pretty much on the edge. An assignment helping out the Assistant D.A. may be the start of a serious relationship but then, tired and disillusioned after a job one night, she finds herself accepting a proposition in a bar that leads rapidly into a web of murder and deceit.