Reporter Rowena Price (Berry) meets her childhood friend, Grace Clayton (Nicki Aycox), who seeks her help in taking down Harrison Hill (Willis), a wealthy advertising executive. Grace gives Rowena her e-mails as proof of their extramarital affair, which Hill recently ended.
A few days later, Grace is found dead, drowned and poisoned with belladonna, and it is also found out Grace was in her first trimester of pregnancy, leading Rowena to suspect Harrison. With the help of researcher Miles Haley (Giovanni Ribisi), Rowena goes undercover as a temp at his company, and enters into a game of cat-and-mouse.
Perfect Stranger contains examples of:
- The Bad Guy Wins: Maybe. Rowena turns out to have been the murderer, and successfully framed one of the murders on an innocent man, apparently getting away with it all in the end. However, it's implied that someone may have witnessed her commit the murder.
- Blackmail Backfire: Rowena murdered her "friend" Grace because the latter was blackmailing her, and she then murders Miles when he tries to blackmail her about setting up Hill.
- Bookends: The film begins with a zoom-out of Rowena's eye, beginning with the inner layers, followed by the retina and then the iris. It ends with another eye zoom-in, only it's from a bystander who saw Rowena kill Miles.
- Foreshadowing: Some of Grace's playful lines to Rowena allude to her witnessing Rowena and her mother burying her father's death as a child."Actions have consequences, Ro. You can't just forget people! Bury them and pretend like they never existed."
"Regards to your mother!"
- Frame-Up: The twist ending reveals that Rowena's intention to follow Hill was to frame him for Grace's murder, presumably because there was much evidence pointing to him and she wanted to do it all for a feasible story.
- Karma Houdini: Rowena turns out to have murdered at least three people and successfully framed one of the murders on an innocent man, apparently getting away with it all in the end. However, it's implied that someone may have witnessed her commit the murder.
- Parental Incest: In her childhood, Rowena was sexually abused by her father, prompting her mother to kill him.
- The Killer in Me: The film somehow manages to do both. Through the film Rowena is presented as investigating the murder of her friend, and that the most likely culprit is Hill, only to be revealed that she murdered her 'friend' (who was blackmailing her) before she successfully pins the blame on the innocent, if slimy-looking businessman, Hill. Oh, and she murders again when Miles tries to blackmail her about setting up Hill. All this would look like the Secretive Killer type... except that several times through the film she's surprised by developments that as the killer she should've known already beforehand, as if she's the Amnesiac Killer type. This is because the twist of Rowena being the killer is actually the result of a last-minute change on the ending to decide the identity of the killer.
- "Ray of Hope" Ending: Rowena succeeds in murdering Miles and framing it like he attacked her and is the killer. However, the final shot is of one of the neighbors looking out his window, implying he saw what really happened and can tell the police.
- Shout-Out: When Bill arrives to see Rowena and Miles, the latter tells Bill to drop the glasses, because he isn't Deep Throat.
- Stalker Shrine: It turns out that Miles actually made one of Rowena that's hidden in a secret room in his house.
- Stalker with a Crush: Miles. He sneaks into Rowena's house and watches her make out with her boyfriend, tries to flirt with her in instant messaging by posing as someone else, and has a freaky shrine dedicated to her in his house. Averted with Rowena, while she's using her romantic charm to get to him, it turns out she was doing it to frame him for Grace's murder.
- Too Dumb to Live: Miles finds out that Rowena, the reporter who was investigating her friend's murder, actually killed her after years of blackmail by the victim. After confronting her, he decides to blackmail the woman who just killed the last person who tried to blackmail her.
- Undercover When Alone: When Rowena is snooping around Hill's apartment looking for evidence of who the killer is and she finds evidence that seems to indicate that it's indeed Hill, she becomes visibly scared, even though it's revealed at the end that she was the killer all along. Apparently this is because the ending was changed to make her character the killer, all logic be damned.
- Wham Shot: At the end, someone is seen removing a vial of belladonna from a deodorant can, meaning that we're currently following the killer. This person closes their bathroom mirror, and we see that it was actually Rowena.