The film opens in 1988 at a Valentine's Day dance at Robert Kennedy Middle School in San Francisco. Jeremy Melton (Joel Palmer), a nerdy-looking young boy, asks four girls to dance with him: Shelley Fisher, Lily Voight, Paige Prescott, and Kate Davies; each turns him down, with the first three harshly: "In your dreams, loser!" (Shelley), "Eww!" (Lily), "I'd rather be boiled alive" (Paige). Kate however rejects him more politely "Maybe later, Jeremy". Giving up, Jeremy turns his attention to another, heavy set girl, Dorothy Wheeler. Dorothy invites him under the bleachers with her and they begin to kiss. However, a gang of bullies show up, led by Joe Tulga; Dorothy calls Jeremy "pervert", claiming that Jeremy sexually assaulted her. The bullies then pour punch on him, chase him under the bleachers, strip him to his underwear, and beat him in front of the crowd of students, who laugh at him, with Jeremy affixing Dorothy with a look of fury as his nose bleeds.Present day, Los Angeles. Shelley (Katherine Heigl), now a medical student at UCLA, is on a date with Jason Marquette (Adam Harrington), an egotistical yuppie. Shelley is already regretting the date and eventually checks out of it to go to the morgue and study for an exam the next day. After being surprised by a fellow student, Shelly finds a Valentine addressed to her, signed "J.M."; the Valentine reads "The journey of love is an arduous trek. My love grows for you as you bleed from your neck." Returning to her cadaver, Shelley is attacked by an assailant wearing a black coat and Cupid mask, who is armed with a knife; finding herself trapped inside the morgue, Shelley is forced to flee into a room full of other cadavers. The assailant follows her inside and starts stabbing the other cadavers to check if she's inside. Eventually, Shelley is found and the assailant cuts her throat, his nose bleeding from the hole in his mask.San Francisco. Kate (Marley Shelton) and Paige (Denise Richards) are walking into a turbo-dating session for Kate, discussing Kate's boyfriend Adam, who suffers from a drinking problem. The turbo-dating session goes just as well as Shelley's date, though Paige gets some action by introducing herself to one of the last guys, named Brian (Woody Jeffreys). Kate receives a call about Shelley's murder. The scene cuts to Shelley's funeral, where we meet the last two members of the girl posse, now grown up: Lily (Jessica Cauffiel) and Dorothy (Jessica Capshaw). The gals are questioned by Detective Vaughn (Fulvio Cecere), who asks them about Shelley, but they hadn't seen her for a year. Another attendee is Adam (David Boreanaz), who reunites with Kate.Returning home from the funeral, Dorothy finds that she's received a Valentine. This one, signed J.M., has a similar message to the one Shelley received: "Roses are red, violets are blue, they'll need dental records to identify you". Dorothy's boyfriend Campbell (Daniel Cosgrove) arrives, saying that he needs a place to stay after his roommate didn't pay the rent, so Dorothy allows him to stay at her house. Dorothy's stepmother Kim (Benita Ha) asks her about Campbell, causing an argument between the two of them before it's broken up by Dorothy's father. Meanwhile, Kate is showering in her apartment when the water runs out, forcing her to rinse her hair in the toilet. After calling the landlord to ask him to fix the water, Kate is lured out into the hallway, where she finds a Cupid mask similar to the one worn by Shelley's killer, and encounters her neighbor Gary (Claude Duhamel). Gary, who mainly speaks in rhyme, asks Kate for a date, completely oblivious to her discomfort around him. Paige and Lily are reviewing a video date sent by a guy named Lance when the doorbell rings. When there's nobody at the door, Paige assumes it's Lance because he's short enough to not clear a peephole. However, there is a box of chocolates on the doorstep with a Valentine from J.M. attached; this one reads " 'tis a well-known fact that beauty is skin deep. Savor the taste, you are what you eat." Lily bites one of the chocolates only to find that they're full of maggots, dropping the entire box in horror. Paige and Lily then try to figure out who "J.M." is, going through various men with those initials before deciding on Jeremy Melton, making fun of his appearance and glibly remarking about how mean they were to him.That evening, Kate, Paige, Dorothy, and Campbell go to a photo art exhibit hosted by Lily's boyfriend Max (Johnny Whitworth), who seems to be interested in Kate. The exhibit consists of a series of maze-like passageways with shifting portrayals of love, with Kate referring to the exhibit as a lingerie commercial. Lily and Max start to make out in a quiet corner, but are interrupted by Max's assistant Amy, looking to join in. A furious Lily attempts to storm out, but gets lost in the maze. As she rounds a corner, she is shot in the stomach with an arrow. The Cupid-masked killer has arrived, and he lines up another shot and fires, this one causing Lily to stagger through a door overlooking a long staircase. The Cupid killer, nose bleeding, fires a final arrow; this one knocks Lily over the railing and she falls 10 stories down into a dumpster, whose lid closes on her. At the entrance to the exhibit, Campbell is confronted by Ruthie (Hedy Burress), who accuses Campbell of scamming her. As the group leaves, Ruthie tells Dorothy "Watch your back, leech".Sometime later, Kate and Adam are sitting in a bar, drinking club sodas. Kate tells Adam that strange things have been happening, referring to the Cupid mask, Campbell being confronted by Ruthie, and Gary leering at her whenever they're in the same room together. Adam asks for another chance, and Kate admits her apprehensions, particularly regarding what happens when he gets drunk. The next morning, Kate, Dorothy and Paige are questioned again by Detective Vaughn, who asks about Lily; Paige replies that Lily is out of town. Vaughn shows the gals a remembrance card sent to Shelley's parents. Inside is a simple message: "Too bad, so sad." This one is signed: Jeremy Melton. When Vaughn chastises them about sending him on a wild-goose chase after Jason Marquette, Paige says she thought the chocolates were a joke and Dorothy and Paige repeat the lie about Jeremy assaulting Dorothy. After meeting with Vaughn, Dorothy admits that she lied to the authorities about Jeremy assaulting her, and that she framed him because she couldn't handle the bullies showing up, citing how fat she was in middle school. While Kate is shocked by what Dorothy had done, Paige tells Dorothy not to worry, stating "Jeremy Melton couldn't manage a water fountain without screwing up. I don't think he's capable of an intricate revenge plot."At her office, Kate does some digging, learning about the original report when she is surprised by Adam. The two of them have dinner and Kate, who tells Adam about Jeremy Melton, offers Adam to come up to her apartment, but he declines, saying he doesn't want to do something he regrets. The next morning, the gals reveal Dorothy's lie to Vaughn and a few more details are filled in: Jeremy was sent to reform school, juvenile hall a year later, and a state-run mental hospital six months after that; Vaughn remarks that they "did a number on him". His parents also died in a fire a couple years before. Vaughn then shows the gals some age-progression software, showing what Jeremy might look like in the present day, since he's been off the grid for a while, with Vaughn throwing in the possibility of plastic surgery. Vaughn then questions them about their significant others; Paige says she doesn't have one, Kate talks about Adam; they met at the same newspaper firm where Adam's a beat writer for the San Francisco Giants, his parents are from Seattle. Dorothy says that Campbell is an internet entrepreneur who she met via yoga class. However, she doesn't know his last name, causing a small argument between her and Paige over who's more careless with men. Dorothy storms out and Kate follows. Vaughn makes Paige stay behind, whom he proceeds to sexually harass before Paige is able to leave.Meanwhile, the Cupid killer sneaks into Kate's apartment building. Entering her apartment, he picks up an iron and finds Gary inside, trying on Kate's underwear. Gary tries to explain that he's "not well", but that doesn't stop the Cupid killer, who strikes Gary in the face with the iron, then burns his face and proceeds to beat Gary to death. Adam shows up at Kate's building, mentions to Kate that he was picked up and questioned by Vaughn, and the two exchange Valentine's Day gifts; an I.O.U. and a heart-shaped lollipop. Kate finds Paige waiting in her apartment (after nearly clobbering Paige with the iron), and after Paige complains about Vaughn, the two of them sit in on a call with Dorothy and Vaughn; Vaughn tells them he's picked up Marquette for questioning, told Campbell to stay put for a few days, and to not go anywhere alone.Dorothy is planning a Valentine's Day party at her mansion. That morning, Campbell and Dorothy exchange gifts; Campbell receives a watch and Dorothy a cupid necklace. Campbell then takes a phone call where it's revealed that Campbell is a con man who wants money to Dorothy's father's accounts; Dorothy then asks Campbell to fix the pilot light. As Campbell works on the pilot light, he is stalked by the Cupid killer, with Campbell finishing his work just before the killer buries an ax in his back, killing him.That evening, Kate brings Adam and Paige brings Brian to Dorothy's party. Dorothy however is in a funk since Campbell is nowhere to be found and she assumes she's being dumped. Brian and Paige go upstairs, but Paige, finding Brian not to her liking, tricks Brian into taking his clothes off first, ties him up and blindfolds him, and pours wax on his penis because he asked her to "wax it", then leaves, with Brian still tied down on the bed. Meanwhile, Ruthie is also at the party and she tries to confront Dorothy, who is wearing her necklace, but is shown out by Kate and Paige. Paige decides she's done for the evening and goes for a dip in the hot tub while Max flirts with Kate, revealing that Lily didn't make it to L.A. and she's been missing for a week, meaning that she's probably dead and that Jeremy might strike at the party.Ruthie, now wandering around the house, winds up in a billiard room. The Cupid killer is there as well, having just finished killing Dorothy's housekeeper Millie. When the killer advances on her, Ruthie knocks him down with a pool cue and tries to run for it, finding Campbell's corpse in a sauna. She attempts to hide in a shower room, but the killer finds her, throws her through a shower door, and impales her head on a shard of glass on the floor. Kate calls Vaughn to tell him that Lily's probably dead; Vaughn reveals that Marquette is clean and that he's on the way to the party. Adam meanwhile has started to hit the drinks table. Kate is furious and tells him to get away from her despite his protestations.Paige meanwhile is relaxing in the hot tub with a bottle of wine when she finds a rose waiting for her at the edge of the tub. When she goes to investigate, asking if the mystery man wants to be her Valentine, the killer appears again, first throttling her, then throwing her into the hot tub and locking it shut. Picking up a power drill, the killer stabs at Paige with the drill, eventually catching her in the shoulder before opening the lid and throwing the drill inside, electrocuting her.The surge of electricity causes the power to go out at the mansion, so everyone leaves except for Kate and Dorothy. Kate and Dorothy start to accuse each other's boyfriends of being the killer, that either one could have had plastic surgery as a disguise, along with some trips to the gym. Dorothy finally snaps and pours out all her resentment over being the loser of the group when it came to men, that she was "always the fat one" before storming off. Kate calls Vaughn again, but hears his voicemail at the house, following the ringtone over to a pond where she finds his decapitated head.After a scream is heard, Adam shows up and he and Kate have a slow dance, with Adam, now sounding more hammered, saying that Kate's the only good thing in his life. A frightened Kate knees Adam in the groin and flees, finding Paige and Ruthie's corpses. Adam follows her, asking her why she wants to hurt him. Telling Adam to stay away from her, Kate strikes him over the head with a bottle and this time grabs a pistol from a locker. As she makes her way upstairs, gun in hand, the Cupid killer jumps out and tackles Kate down the stairs. As both of them come to, Adam reappears with Kate's pistol and guns down the killer. As Kate apologizes, Adam removes the Cupid mask to reveal Dorothy. After Adam calls the cops, Kate is in shock, admitting she doesn't understand why Dorothy would murder all those people. Adam replies that he believes that Dorothy was so lonely and angry that despite the friends she had, how happy she seemed, the anger and resentment she felt never left until she decided to do something about it. The two of them say that they love each other and share a hug. As they sit, blood starts to drip onto Kate's cheek and the camera pans up to Adam, whose nose is bleeding. This reveals that Adam was a cover identity for Jeremy Melton, who had committed all the murders and framed Dorothy for his crimes to exact his revenge.