Sharon's end-of-summer vacation with her family turns disastrous when her dad, Richard, introduces them to his new girlfriend - Nina Harper's mother! Sharon can't believe that there's even a remote possibility that Nina could end up being her step-sister and she does everything in her power to prevent it. Unfortunately, Nina doesn't mind the situation as much as Sharon does. She sees this as the perfect opportunity to get close to Adam. Step-siblings can date, can't they?
The harder Sharon tries to convince her family that Nina is baggage they don't want, the more Nina endears herself to them. Sharon is finally convinced that she must resort to Nina-like tactics to prove Nina's innate evilness. An attempt to frame her for stealing Adam's championship wrestling ring backfires and Sharon is left bearing the "bad girl" reputation. For Sharon, the entire holiday is her worst nightmare come true…
…until she wakes up in the orthodontist's chair and realizes that it was all, indeed, just a dream. Suddenly, Dr. Hertz's announcement that she will need to continue wearing braces throughout high school no longer seems like the worst fate a girl has to endure.