Again with the possible SPOILER ALERT!
I'm sure the poster speaks for itself but to elaborate, Beyonce (Sharon) and Idris (Derek) are married, Ali (Lisa) is a temp at the company where Derek works and becomes so delusional that she thinks she is having an affair with Derek. She starts following him and getting into his car in nothing but a trench coat and lingerie. She follows him to a conference and plays it off like she's his wife (and the people at the front desk of the hotel don't know any better so of course they think she's his wife) she tries to OD on pain pills in his room (again the poor people at the front desk have no clue what's going on and give her a key to his room cause "she's his wife and wants to surprise him" PSYCHO!!!) She goes into their house and tells the babysitter that she's a friend of Sharons and that she's just dropping a present to the baby (Kyle) and of course plays it off so well that the poor babysitter has no clue and Lisa takes the baby. She doesn't take him far she puts him in his car seat in Dereks car with a big red kiss mark on his forehead. As Sharon is getting ready to leave for her parents wedding anniversary party and Derek is at work Sharon forgets to set the new alarm to the house, she goes back to set it and Lisa is in the house. She's decided that she and Derek are having a rendezvous and "they" didn't want Sharon to find out this way. Lisa really is convinced that Derek her husband or they're lovers or some such psycho babble like that and Sharon is out of the picture. Lisa dies at the end cause Sharon goes up to the bedroom and finds her and beats the crap out of her, she ends up falling through the ceiling and a chandelier falls on here and kills her.
The lesson here I'm not friends with many woman cause we're all crazy!!! Kidding, but really creepy that there are people out there that get this way. Women are NUTS!