Josie and Rachel are twin sisters who look and act nothing alike. They and their best friend Melissa go horseback riding with their little sister Erica who is about to be a freshman at Shadyside High with them. Erica chickens out at the last minute and refuses to go riding. Rachel is fine with it, but Josie keeps snapping about how she's so lame and such a baby. They're in such a hurry to hit the trail that no one checks Rachel's saddle. A dog runs out in the middle of the trail, her horse rears up, and she falls off and lands on her head.
Part One: The Following February
It's been a few months and Melissa still isn't over what happened. She keeps having nightmares about that day, and she tells her mom that Josie blamed her for what happened. Not only do they rarely talk now, but she's also dating Josie's ex-boyfriend Dave.
Then we go over to the twins' house. Erica now gets stuck watching over Rachel, who has the mentality of a kid, all the time. Josie constantly makes up excuses for wanting to hang out with her boyfriend Steve or doing other things so she doesn't have to stay with Rachel. We also find out that Rachel's boyfriend Luke hangs around all the time even though she doesn't recognize him.
Josie gets a secret valentine in the mail that pretty much wishes her dead. Steve laughs it off, so she shows it to Erica. Before Erica can really say anything, Josie takes off again. The next day, Josie once again skips out on watching Rachel. Erica has an important tryout, but Josie doesn't care. Erica blames her for the accident right before someone nails her in the head with a snowball. You would think that's someone trying to tell her something, but she just runs off for another date with her boyfriend.
When she gets home that night, she finds Luke there. Luke pretty much calls her out on her crap. He tells her that she's conceited, selfish, and doesn't care about anyone else. He points out that Erica is the only one caring for their sister because her parents are apparently never home and that the audition was really important to her. When Josie snaps back at him, he stabs her through the heart with a letter opener. I wish. He actually just threatens her with it before slamming it into the desk and warning her that he almost did it. It doesn't help that she then gets yet another valentine in the mail.
Late one night, Josie hears Rachel calling her name over the intercom system in the house. Though she tells her to go back to sleep, she keeps calling for her. Josie sneaks into her room and finds her sound asleep. She later hears her calling her name again, but she once again finds her asleep.
Josie clearly doesn't care about making friends or keeping her old friends. She tells her teacher that Dave, Melissa’s boyfriend, cheated off her test. The teacher gives Dave a zero without you know finding any proof that he cheated. Dave ends up kicked off the wrestling team and loses his shot at a scholarship for college. Now we have one big suspect.
Back at home, Rachel calls for her over the intercom and Erica lets her know that their sister wants her. Instead of talking to them, she runs out of the house and meets up with Steve. They go ice skating, and she sees someone watching them. She shows Steven another valentine she got, but he laughs it off and acts like it's nothing. I would suspect him, but I've read this book before! Josie comes home later that night and found that someone killed their dog. This seems like it should be a big thing, but once the police comes, it really never gets brought up again.
Erica and Josie go shopping for valentines. They see Jenkman, Josie's old boyfriend. She thinks that he's the one sending her the valentines. Erica gets a little blushy and makes it cleat that she has the hots for him. They talked on the phone, he admitted to sending her some funny valentines but denied sending the threatening ones, and she thinks he's innocent. When he tries to talk to Josie and she flips out on him, he looks so mean that Erica starts to suspect him.
Josie starts talking about the valentines she got, which leads to Rachel singing about how someone hates her, someone really hates her. Yeah, and it's me. She storms out of the house despite Erica trying to call her back, claiming that she'll watch her sister the next night.
Erica wakes up after a nap and realizes that her sister still isn't home. She calls Steve, who admits that they had a fight and Josie took off with another group of kids but she should be home by now. Erica hears someone at the door and drops the phone, but it's the cops. They tell her and her mother that they found Josie's dead body earlier in the night.
Melissa gets a phone call from Dave telling her that he ran away. She runs off to meet him, and he tells her that he was the one who sent the valentines but that he didn't kill Josie. He decides to sneak into their house, find the valentines, and throw them away before anyone sees them. He finds the house completely empty and stumbles on something slick. Reaching for his balance, he accidentally grabs something in his hands. The cops suddenly show up and see him standing over a body with a bloody letter opener in his hands. It's Erica and she isn't moving.
Part Two: February, One Year Later
It's now one year later. Erica survived the attack but spent a lot of time in recovery, and her parents still expect her to take care of Rachel all the time. Melissa and Luke are now dating, but when Luke stopped going to see Rachel, she had some kind of a setback. Oh, and Melissa got a bad valentine too. Dave, who probably didn't kill anyone, got a bad reputation and his parents sent him to a military boarding school. He sent her a letter, which brought back all the bad memories in her head.
While talking to Rachel, Erica is shocked when her sister asks for Luke. She reminds her that he's now with Melissa, which leads to Rachel screaming that she hates her former friend and attacking a teddy bear Luke gave her. The phone rings, and it's Steve asking her out despite not talking to her once after her sister's murder. He tells her that she deserves to have a life and hangs up. After getting off the phone, she realizes that her sister is missing. She finds Rachel outside making cryptic comments again.
Cut to Melissa and Luke. She got another valentine and wonders if Dave is behind it. They make plans to find the originals and see if the handwriting matches. Before they can make any further plans, the phone rings. It's Dave's mom calling to tell her that Dave ran away from his boarding school. He's been talking a lot about the last year and about Melissa, and she's worried he's coming home.
Erica hears weird noises a few nights later and finds Luke sneaking around their house. He says he just wanted to drop off a valentine for Rachel and disappears again. Dave shows up at Melissa's house and tells her that he finally figured out who the real killer is and that he wants to clear his name. She asks him for more information but he runs away.
She then goes to see Rachel. Since Erica had a play rehearsal, she agrees to watch her for a few hours. Rachel says she likes to go outside and makes some more weird comments. When Erica gets home, she finds that someone sent her a threatening valentine, which leads Rachel to singing that someone now hates her. She also says she likes going outside and goes out by herself, and when Erica says she isn't allowed outside by herself, Rachel goes quiet. Melissa wonders what the hell is going on when she sees someone who looks like Rachel running across her lawn in the middle of the night.
Since the weird noises she heard also wake her parents, they all try to figure out what happened. Melissa makes it clear that she saw her former friend but they say it's impossible. The police then randomly show up and think they called because of what they saw outside. They ask about the body of the teenage boy in the driveway, and Melissa starts screaming about Luke.
The police go to Erica's house and tell her parents, who are actually home for the first time in this book, that they need to talk to their daughter. They say that they need to question Rachel about Dave's murder, so we now know who was in Melissa's driveway! Their parents say it couldn't be Rachel because she isn't allowed outside by herself, but Rachel smiles and says she goes outside alone all the time.
A few days later, everything is kind of okay again. The police realized Rachel couldn't be a killer. Melissa seems to have gotten over her ex-boyfriend's death fairly quickly and is looking forward to moving on at an ice skating party with Luke. Erica plans to go too, so I guess her parents are finally letting her out of the house. Melissa stops by her locker and finds a bloody note warning her that she's next.
No worries though. She ends up at the party with Luke. Someone freaks out because the ice is cracking, but that doesn't bother Luke. He pulls her to a secluded place and gets little rapey but then settles down and tries to kiss her. Since he's such a good skater and she isn't, she tells him to go have fun and let her practice a little. As soon as he leaves, she realizes that the lake is the biggest lake in the world and that she's miles away from the other guests.
She notices someone skating quickly towards her and then sees a knife in the skater's hand. When she sees red hair, she thinks it's Rachel, but it's really Erica. Erica stabs her, but since she's wearing her dad's heavy coat, it misses her. She then flips out and screams about how she did it all for Rachel and to pay back everyone who hurt her. She also says that she saw Dave sneaking into the house that night and stabbed herself to frame him and that she killed him when he figured it out.
The two of them start fighting over the knife. The ice cracks and they both go under, but Luke gets to her and pulls her out of the water. They keep looking for Erica, but then they see her body under the ice. Luke realizes that she doesn't want to save herself.
At the end of the book, they both go to see Rachel. She talks about how much she likes school and drawing and that she wants to show her drawings to Erica. She also says that she wants to get better and go outside by herself. They wander off and think about everything that happened and decide that instead of celebrating Valentine's Day, they'll just celebrate Groundhog Day from now on.