Since her husband died Lianne (Arianne Zucker - Killer Assistant) has been extremely busy with her work, so busy that at times her daughter, Summer (Ava Locklear), thinks that her mum's work is more important than she is. So when Summer befriends the new girl in school, Cara (Sierra Pond), Lianne doesn't pay a great deal of attention, although notices how nice Cara is to her, almost a perfect daughter. So when Summer goes missing Lianne finds herself beginning to become suspicious of Cara especially as she seems determined to be the perfect replacement daughter.
I don't think "Mommy Be Mine" had even made it to 20 minutes before it had revealed everything. What I mean is before 20 minutes are up we learn through some not to subtle clue dropping that Cara has mommy issues, as in she wants a mommy, and is quick to work her into the home life of Summer and then getting close to her mom Lianne. Of course whilst all this is going on we have Summer's friends who immediately take a dislike to the way Cara is always around and that means they need to watch their backs, especially her boyfriend. And just for good measure Lianne finds herself dealing with a work colleague who likes her, leading to her wondering whether she could be ready to date whilst we wonder whether he will get to be the hero of the movie or another unfortunate victim.
What this means is that "Mommy Be Mine" ends up routine as we watch the twisted Cara go about manipulating situations, getting people in trouble, and becoming more and more psychotic, watch out for a cupcake scene you would not want to be a cupcake. The trouble is for the sort of audience who watch movies such as "Mommy Be Mine" all of this will be too familiar with the only difference being that we have an unhinged teen who wants a Mom's love and will do what ever they can to get it. In fairness Sierra Pond is pretty decent as Cara but even then it still ends up familiar at best.
What this all boils down to is that "Mommy Be Mine" ends up too familiar for its own good which means for fans of these sorts of made for TV movies it ends up all a little too familiar and predictable.