Quinn Hadley is a freshman at Salem University and a chronic night walker, aka sleepwalker. She’s had a few episodes in her life and the doctors always told her that it was just stress. She was also warned to watch herself at college because school was so stressful. When her boyfriend Simon dumps her right before the Spring Fling, she starts sleepwalking again.
Her roommate Tobie keeps an eye on her and keeps her from doing dangerous things when she sleepwalks. Tobie and some other girls from her dorm go to the dance together and everyone has a great time until the last dance. Suddenly an awful smell takes over the room and everyone panics, running outside. Several people are hurt and her friend Jessie gets her wrist broke. Someone apparently did a little chemistry experiment and set off sulfuric acid in the room.
When she wakes up the next day and hears about it, she notices a sulfur smell in the room. At first she thinks it’s Tobie’s clothes, but then finds that it’s her own clothes. She quickly hides everything away, but worries that she did it in her sleep. Apparently she had two bad episodes before. Once she had a fight with her sister and sleepwalked into her room, where she beat the crap out of her. She also attacked a girl’s tennis racket in her sleep, after the girl beat her.
Later that day, she tries to talk to Simon about their breakup, but he acts really distant and runs off. Quinn decides to start wearing clean socks to bed every night, to keep track of herself. A couple is attacked that night, when someone throws red paint on them from a balcony. Quinn checks her socks, which are clean, but then finds her clothes covered in red paint.
She does however make up with Simon. She confronts him over their break up and he shows her a letter that he got, signed with her name. Someone sent him a letter on pink stationary, breaking up and telling him that they didn’t want anymore contact. They make up and start dating again.
Then another couple is attacked. This time the attacker smashes their car doors with a hammer, when they’re parked together. Apparently it’s 1958 at Salem University. Then the attacker battered all the windows and raced off. Quinn discovers that her socks are covered with dirt and mud and so is her yellow rain slicker and hat. She visits the site of the incident and sees her friend Suze poking around.
When she pulls out a pair of jeans, she finds shattered glass in one pocket, exactly the same size and shape of the car windows. Suze claims she visited the site as a favor to Reed, the girl who was attacked, but Reed denies it later. She also makes a comment about how the attacker wore a yellow rain slicker and hat.
Quinn’s friend Tim also has an interesting story about Tobie. Tobie dated a few guys on campus, but never settled down. She tells Quinn that she has no interest in a relationship or being with one guy. Tim reveals that Tobie had a high school sweetheart. They were in love, made plans to go to college together and even talked about marriage. He was in a car accident the summer before college and died. To make things even worse, Quinn finds pink stationary in Tobie’s desk, just like the letter sent to Simon.
Instead of confronting her, Quinn opts to go out with Simon. Their friend Ivy is pissed because they’re skipping out on her sometime boyfriend Tim’s frat party. The restaurant and movie are both deserted because no one wants to be seen as a couple. The party turns out the same way. When she gets back, she finally confronts Tobie. Tobie denies writing the letter and tells her the story of what happened. Her boyfriend Peter was actually taking money out of an ATM, when he was attacked. He fought back, the attacker pushed him, he hit his head and died.
That same night, another couple is attacked. This time it was Ivy and Tim and they were attacked by a claw hammer. Quinn is shocked to discover that the claw hammer is hidden under Tobie’s pillow. She takes it to the police and finds an article about Peter’s death. The cop tells her that Tobie’s parents sent it to him, when all the weird stuff started happening. Apparently someone was with the attacker at the time, but he never gave up a name. A plus-size blonde came to the proceedings though and glared at Tobie when she testified against him.
Quinn finds out that the guy’s name was Gunther and that his girlfriend’s name started with an S. Simon is attacked and knocked out, which makes things worse. She goes to Ivy, wanting to tell her that she suspects Suze. She finds a ring on the table between the two beds and plays with it, while telling her what she knows. Then she reads the inscription and sees that it’s from Gunther! And it belongs to Suze! She and Ivy hatch a plan to trap the girl, but then Suze shows up.
While Ivy is in the bathroom, Suze warns Quinn about wearing the ring, telling her that Ivy is super protective of it. Quinn freaks out because that means Ivy was his girlfriend. She runs off, but Ivy follows her and nearly runs her off the road. She wrecks her car, jumps out and runs to Nightmare Hall, where she hides in the barn. Ivy follows and Quinn runs up to the hayloft.
Ivy explains that she dyed her hair, got contacts and lost weight to completely change her appearance. She and Gunther planned on running away together and he attacked Peter, trying to steal money for them to run away. He loved her so much that he kept her name out of the case. Ivy sprinkles kerosene all over the barn and then climbs up the ladder with a lamp. Quinn pushes the ladder over and when Ivy hits the ground, the kerosene catches fire and the barn goes up in flames. Quinn survives, but Ivy dies in the fire.
Quinn winds up in the hospital, where her friends talk about what happened. Tobie confesses that she’s now seeing a therapist and while she’s not ready to start dating again, she feels good about her progress. Suze confesses that she went out with the male from the second couple and left her psychology book in his car, which is why Quinn saw her there. She also explains that she found her sleepwalking once and brought her back, which is why her socks were dirty. Quinn realizes that Ivy kept doing the stuff, like splattering her clothes with paint, to make Tobie look guilty because she thought it was her stuff. She’s also happy and relieved that her sleepwalking days are behind her.