When a teacher using a dead woman’s identity sets her sights on a student’s father, she becomes a threat to the family when she doesn’t get what she wants… him.

Ally starts her first day of high school as a junior with her best friend Layla. Their History teacher, known as Michelle Langford is actually Tiffany Harris, a deranged woman who has killed the real Michelle and taken her identity.

Ally’s dad, Lucas, waits outside the school to pick up his daughter. When Tiffany gets a look at the very attractive man, she makes up an excuse to introduce herself. That doesn’t seem to get Lucas’s attention so she fails Ally’s History report in order to call Lucas to discuss it while allowing her student to re-do the report for a better grade. The two go out for coffee and hit it off.

Later when Lucas arrives at home, Ally questions him and discovers that he was having drinks with her teacher. This makes Ally very uncomfortable and she asks that he not date her teacher. With Tiffany being somewhat pushy, Lucas finds himself taking her out on a date, then having an intimate encounter. Ally panics, fearing that fellow students will think she’s getting preferential treatment from Tiffany. Lucas offers to stop seeing Tiffany if it’s going to upset Ally that much. Loving her dad more than anything, Ally decides to step back and allow the relationship to continue, even though she still disagrees with it.

One night while Lucas is sleeping over at Tiffany’s apartment, Paul, the husband of the dead woman, has traced his wife in name to Tiffany. When she opens the door, Paul is disappointed but also adamant that it’s his wife living there. Trapped in a corner, Tiffany claims to be Michelle’s roommate, who isn’t home right now. She will text Michelle and have her meet her husband at a remote location in an hour. Paul waits at the scenic overlook for his wife, only it’s Tiffany who arrives, driving his wife’s car. She claims that she couldn’t get Michelle to come while distracting Paul. A sudden push sends the threat to Tiffany’s true identity over the edge of the cliff, to his death.

Ally starts dating Brad, a student in her History class. She invites him on a weekend getaway with her dad, who has invited Tiffany to come alone. Disapproving of Brad, Tiffany attempts to harm him in a boating incident while making it look like an accident. Tension breaks out between both couples and they leave the lake cabin early.

While snooping on “Michelle”, Ally discovers who she really is. Tiffany knocks her out and stuffs her in the trunk of her car. She gets Lucas to ride with her back to the cabin, where she drugs him and attempts to drown Ally. Lucas forces himself to throw up the drugs and rushes to save his daughter, getting shot by Tiffany in the process. All three end up in the water, and Tiffany can’t swim. Lucas focuses on saving the most important person in his life, allowing Tiffany to drown.