Kelli is a beautiful 18 year-old and recent high school graduate whose setting out to enjoy her last summer before the responsibility of the real world sets in. Kelli's parents, Holly and Sean, 40's, are minutes away from leaving on a weekend trip, leaving Kelli home alone for the first time under a strict warning that visitors are not allowed in the house. Kelli's leaves for a friend's house while her parents finish packing.
At her best friend Tess's house, Kelli reveals that she plans on giving up her virginity to her boyfriend Corey this weekend. This shocks Tess, because they both purchased promise rings together, and now Kelli is breaking the pact. Tess is disappointed but supportive.
Kelli returns home from Tess' house, unaware her parent's are dead in the master bathroom upstairs and The Killer is still in the house. Kelli goes about her night enjoying her freedom. She texts Corey, eats pizza, drinks her dad's beer, looks over what she's going to wear for Corey. It is all about the big night for Kelli, and hidden in darkness, The Killer studies his prey watching her every… single… move.
Later that night, Corey arrives late. Kelli is pissed, but Corey is too much of a player and quickly wins her back over. After starting in Kelli's room, Kelli moves the fun to her parent's more luxurious bedroom. Corey proceeds to steal her innocence while The Killer watches from a nearby closet. Afterwards, Corey, the "typical jerk-of-a guy," shows his true colors, after all… he got what he came for. They argue and Kelli throws him out. Corey could careless and he leaves, luckily escaping The Killer's wrath.
Upset, Kelli texts Tess for support. Tess offers to come over but Kelli wants to be alone. Crying and confused, Kelli breaks down and calls her mother Holly. Holly's cell rings throughout the house and Kelli runs around looking for it. She runs for the master bathroom and behind her, The Killer follows… The Killer yanks Kelli by the back of the hair tossing her hard to the ground. She's dazed trying to fight back, but she's no match and The Killer begins a music-video-style super-slow-motion slashing of Kelli. Kelli's body falls dead next to her mother and father's bodies.
The next morning, Tess arrives looking for Kelli. There's no answer and she lets herself in and walks upstairs into Kelli's room. She calls out for Kelli and then walks into the master bedroom. Unable to find Kelli, Tess decides to call her cell… Kelli's cell rings lying in a pool of blood a few feet from her hand, the display reading: 'TESS' Tess looks in the master bathroom and sees the massacre.