Casey finds her room in the new house too small to live in so she manifests her dissatisfaction with the parents. It turns out in a battle between the two "step" siblings to determine who will have Derek's room.
Derek manages to insult his new step-sister, making Casey clumsily fall down at school and earn the nickname Klutzilla. And it just keeps getting worse as Casey continues her klutz streak. Finally Derek realizes something, besides his rude behavior, that might be making Casey fall and decides that helping her reputation will help his, as it's bound to get out that they are newly step-sibs.
Casey is sickened by the men of her new home. Between the smell and the disgusting habits the boys have grown to have, Casey feels like she and her family don't belong in their new house. Casey convinces her mom, Nora, to redecorate the house so it feels more at home for her and the rest of the girls. Naturally, the boys are extremely upset and the two families cause a fight between George and Nora.
Casey scrounges for every grade point she can, no matter how high her grade is. When her best friend, Emily, tells Casey that grade grubbing and it's not a good thing Casey tries to lose her new rep. Derek isn't allowed to fail another grade and tries to use Casey's grub rep to get an A, while Casey is using Derek's rep to get a bad grade.
Derek gets the chicken pox which keeps him in quarantine and while he has the chicken pox he is doing something very sneaky. Casey figures it's drugs and finds our he's running a business and selling CD's and what not.She helps him out and starts helping him sell, which gets her in trouble. Meanwhile George is trying not to get sick from Derek.
Casey is sick of having no room in the Venturi house and brings up the subject to Derek and the rest of their siblings. Derek, who doesn't like sharing or losing, tells Casey how they get things in his house. After handing out marbles to represent the disputed areas in the house they start challenging one another for the other marbles. Neither side plays fair, and both sides will be underestimated.
After a clumsy mishap, Casey sprains her ankle and black mails Derek into giving her whatever she wants. But when she gets his concert tickets Derek wants revenge so he comes up with a plot to get Casey to hurt him so he can black mail her. Both turn out bad and hopeless when Marti decides to make a "picture" with the tickets.
After Casey is offended by Derek and Edwin playing Babe Raider (a spoof on the game Tomb Raider), she makes a manifesto stating that they can't play Babe Raider in the house. However, after furthering her research and playing the game, Casey finds that it's not such a bad game after all. Derek is mad about having to take down his posters and feels that he and Edwin are being "brainwashed".
Casey and Derek's best friend, Sam, like each other. When Derek gets the feeling Sam wants to ask out Casey he conveniently lets Sam know about the Male Code which states siblings are off limits and step-sibs count. George and Nora don't think they know their kids.