Idea for a Hitchcock movie?
A serial killer is stalking the city. A woman sees one of the murders but knows she's not the only witness: another man from her apartment building was also there. He, however, lies to the police that he knows nothing. She becomes suspicious of him, even as the police are becoming suspicious of her, as they know something the public don't, that the killer is a woman. Fortunately the suspicious man is, in fact, an undercover agent, tracking the killer who is actually a spy cleaning up a ring of informants. He subtly ropes the woman into helping him track the killer, even as she is investigating him and the police are investigating her.