In the town of Springville on Halloween, two high school students, Shelby and Becca, are at Shelby's home going over the first aid test for their phys ed class. Becca half-jokingly points out that the teacher, Mr. Drake, has the hots for Shelby. They talk about how Becca will be going to Taiwan with her parents, and Shelby comments on how Halloween is the most important date on the Wiccan calendar. Becca suggests that they perform a séance and ignores Shelby's warnings that it can be dangerous. Her friend persists, mentioning Shelby's dead mother, and Shelby finally agrees to make a spirit box.
Shelby, a Wiccan has a box on putting together the spirit box. The two friends assemble it from a discarded pizza box, symbols drawn in their blood, and a pointer made from a cell phone. They use it like an Ouija board and are surprised when pointer suddenly moves, saying "Yes" when they ask if there are spirits present. Next the pointer spells out M-L-E and they realize it's Emily Sarah D'Angelo, a girl from their school who recently killed herself. When Shelby asks it to confirm how it killed itself, the spirits spells out M-R-D-R. However, when the girls ask Emily who killed her, the candles go out and the pointer moves to a new phrase on the box, L8R.
Becca knew Emily as a fellow cheerleader and the two girls check the newspaper report on the girl's death. Emily apparently herself by drinking and then driving into a lake. Shelby and Becca wonder hwy Emily didn't contact one of her friends, since neither one of them new her. Shelby's father, Sheriff Dan Johnson, arrives home before they can ask further question of the spirit. He tells them to break it up since Shelby has to go practice at the swimming pool the next morning.
Early the next day, Shelby is at the pool practicing by herself. She catches brief glimpses of something moving around and reaches a hand back, almost touching what appears to be a ghost's foot. When Emily turns, however, there's nothing there.
Later in gym class, Drake is teaching the kids first aid. Shelby tells Becca what happened but her friend dismisses it as a pervy janitor. However, Becca admits that she kept receiving the L8R = Later message on her cell phone and shows it to Shelby. She puts it away when Drake comes over to check their work. Once he leaves, Shelby worries that they've unleashed the spirit on the world, and Becca suggests that they help Emily get revenge to put her at peace
That night, Shelby and Becca go to the river where Emily drove in and find the shrine left in her memory. Sheriff Johnson investigated the supposed suicide and had learned that Emily supposedly texted her suicide note to her friends. However, they figure that the killer could have sent the fake note. When they realize that they forgot the spirit box, Becca goes to get it and Shelby washes her hands off in the river. A hand reaches up for her and she scrambles away, running off past Becca.
A few days later at school, Shelby tells Becca that she stole her father's official file on Emily's death. The lab report shows that besides alcohol, there was a prescription sedative, barbsieconyl, in the dead girl's bloodstream. Shelby points out that it makes no sense for Emily to overdose herself and then drown herself in the river, and figures that someone drugged Emily and shoved her car into the river. Becca goes to talk to another cheerleader about Emily and Drake comes over to talk to Shelby. He notices a photo of her mother in her locker and mentions that he graduated with Shelby's mother. Drake tells Shelby that she looks a lot like her mother. The class bell rings and Drake leaves, and Becca comes back over. All her fellow cheerleader knew was that one student, Christian Bosch, was obsessed with Emily and even carved her name into his leg. Christian was expelled from school and nothing more came from the incident.
At home, Shelby is looking at photos of her mother when Sheriff Johnson comes home from work. He comments that he's meeting with his sister-in-law and Shelby wonders if they're romantically involved. Sheriff Johnson insists that there's nothing romantic to it and tells her to take out the garbage. Outside, Shelby sees the police computer in her father's car and gets an idea. The door is locked so she goes back inside, takes her father's keys while he's in the shower, and then looks for police info on Christian.
The next morning, Shelby goes to the pool to swim laps but a figure in a park and a tribal bird mask is lurking in the stairwell. She runs back to the locker room and hides in a bathroom stall. The bird-figure follows her and goes to the stall door, but then puts a piece of paper on the floor and walks away. When it's safe, Shelby comes out and reads the paper, which says, "Let the dead sleep in peace." Shelby reports the incident to his father but he finds nothing conclusive. Sheriff Johnson wonders about the note but Shelby lies and says she doesn't know what it means. She suggests that Christian is responsible and Sheriff Johnson has him picked up, but quickly confirms that the teenager has an alibi. He has no choice but to release Christian and wonders if someone is playing a joke on Shelby, but his daughter insists it's not a joke.
Later, Becca talks to Shelby at cheerleading practice, with Drake supervising. Shelby explains that her father would never have believed her. When Drake calls Becca back to practice, Becca clutches at her stomach. He tries to help her but she yells at him to keep his hands off of her. As Shelby takes her to the nurse, Becca starts vomiting and finally throws up a charm bracelet with Emily's name on it.
That night, Shelby and Becca go to the cemetery and place the spirit box on Emily's grave. Emily's spirit speaks to them again, spelling out M-R-D-R. However, it then continues, adding A-K-E, and they realize that it is identifying Mr. Drake as the murderer. A groundskeeper notices them and they are forced to run away.
When Shelby returns home, she finds her father waiting for her. Sheriff Johnsons wonders if she's been out drinking or taking drugs, and Shelby angrily denies it. She finally admits that she was at the cemetery with Becca conducting a séance. Sheriff Johnson assumes that she's trying to contact her dead mother, but Shelby tells him that Emily is the spirit and supposedly committed suicide. When she accuses Drake and mentions the barbsieconyl, her father realizes that she accessed the police files. Disgusted, he warns her that he could lose her job if anyone finds out and grinds her.
The next morning, Shelby is practicing and drops her goggles. When she swims down to get them, Emily's spirit reaches up through one of the circulation grilles. Later, Shelby finds Becca and tells her what happens, and Becca reveals that she's wearing a cast because she injured her knee during cheerleading practice. Becca tells her friend that an unseen force pulled her down during a routine. Drake comes up to them, purportedly to check on Becca, but is more interested in Shelby. Once he goes, Becca says that they have to find evidence so that Sheriff Johnson will reopen the case, and shows Shelby that she managed to obtain the key that Drake leaves out for the maid.
Becca and Shelby drive to the house and Becca gives her friend a taser. While she keeps an eye out for Drake or his wife, Shelby goes in. As she goes upstairs to the bathroom to look for the barbsieconyl, she sees Drake's collection of tribal bird masks. The cell phone connection fades in and out, and Shelby doesn't find the barbsieconyl in the medicine cabinet. She searches the bedroom and is startled when she finds Drake's first aid dummy beneath the bed. Shelby finally finds the barbsieconyl in the nightstand. Meanwhile, Becca spots Drake driving up to the house but is unable to make contact with Shelby, so she calls 911 and reports a break-in.
Shelby hears Drake coming upstairs and hides in the closet, but she gives herself away. He demands to know why she's there and Shelby tries to flirt with him. When Drake responds, Shelby tasers him and he falls backward down the stairs. Shelby checks on him and discovers that his neck was broken in the fall, but he's still barely alive. Becca arrives and assures her friend that the police are on the way. She tells Shelby to go outside and wait for them, and then looks down at the dying Drake.
One month later, Shelby goes to leave flowers on Emily's grave. However, she notices that the gravestone is loose and priest it up, revealing a different gravestone beneath. Shelby calls Becca in Taiwan and tells her what she's discovered, noting that Becca said she attended Emily's funeral. Becca finally admits that Emily was cremated and she sets the whole thing up. Becca got her fellow cheerleader drunk and then faked her suicide and sent the text messages. When Drake still ignored her, she gave Shelby the idea of the séance, faked Emily's messages and the injured knee, fake-vomited up the charm bracelet, planted the barbsieconyl in the nightstand, and sent Shelby into Drake's house. However, she admits that she had nothing to do with what Shelby saw at the pool, but figures that her friend imagined it.
When Shelby objects, Becca tells her that there's nothing she can do since Shelby "proved" Drake killed Emily. Even if Shelby does go to the authorities, Taiwan has no extradition treaty with the U.S. When Shelby wonders why she did it, Becca tells her that she and Drake were lovers, but he dumped her for Emily.
Becca hangs up and goes out to the garage to start up her car. Once she's inside, the garage door refuses to open. The doors lock, sealing Becca in, and the car starts up. As exhaust fumes flood the garage, Becca tries to call for help. However, the L8R/Later message repeats over and over on her phone. As Becca looks down in horror, Emily's waterlogged corpse lunges up from the floor. The spirit then walks away, leaving watery footprints, as Becca chokes to death.