Short, very predictable version:
Lisa (Ali Larter), the stalker, gets killed in the end. Sharon (Beyoncé Knowles), Derek (Idris Elba) and little Kyle survive and are safe.
Long, drawn-out and still very predictable version:
In a last attempt to have Derek for her, Lisa breaks into Derek and Sharon's place one last time, upon confirmation from Patrick (Matthew Humphries) that they would be gone for the weekend. This is possible, thanks to Sharon failing to put the alarm while trying to collect her purse on her way out; however, she remembers not having done it and drives back. Meanwhile, Lisa breaks through the back door, goes upstairs, strips and wears a jersey shirt, draws rose petals on the bed and pops a bottle of champaigne -this is heard by Sharon, as she had timely returned to set the alarm-; Sharon realizes what is going on and goes upstairs, finding Lisa laying on the bed, fantasizing. They get into an argument which escalates into the very expected showdown in which they fiercely kick each others' butts.
They both wind up in the attic, where Sharon lures Lisa into going through the defective, unstable floor (remember, at the beginning, when she complained about this?) and dangling off it. Sharon has a change of heart and tries to help her up, but then, the very deranged Lisa tries to pull her and Sharon rips her off her hand. Lisa falls and smashes the glass coffee table. Is she dead? NOOOO. She opens her eyes; but then, the hanging chandelier, weakened from the ceiling being torn apart, swings and lets loose, landing on Lisa and finishing her off for good.
Bloodied and staggering, Sharon comes out of the house, as Detective Reese (Christine Lahti) arrives asking what happened; she firmly answers: "You know what happened!"; at the same time, Derek arrives and they hug and kiss.