Casey finds her room in the new house too small to live in so she asks the parents to have Derek's room. It turns out in a battle between the two stepsiblings to determine who will have Derek's room.

Casey trips at school on the staircase (and takes it out on innocent bystanders) after Derek insults her. Derek can't stop laughing as he and his best friend, Sam, look on. Derek nicknames her "Klutzilla", a name that quickly spreads throughout the school. The whole school is "watching out" for Klutzilla and she blames it all on Derek. A second incident happens when Derek insults her again and she falls into Marti's box in front of Sam. Soon, Derek connects the two incidents to Sam and accuses Casey of secretly crushing on him.

Casey and Derek must persuade their parents they can be left alone for the weekend. Casey intends to have a quiet weekend at home and watch some movies with Emily, but as always Derek has conflicting plans and has a party and damages the house.

Living-room redecorating creates strife within the family, as Casey, Lizzie, and Nora want to add a feminine touch to the household, while Derek, Edwin, Marti, and George are perfectly fine with the way the household is.

Casey discovers she has a reputation as a grade-grubber and wants to rid herself of it by choosing a slacker to work with on a group assignment – like Derek. Derek is on the brink of failing English and wants to pick someone smart to work with on the same group assignment – like Casey.

Casey and Derek volunteer to organize Casey's aunt's wedding reception, but Casey's cousin, Vicki, is threatening to blow the entire thing apart because her mom didn't even ask her how she felt about the marriage.

Derek is quarantined at home as he is with the chicken pox, and is hiding a secret which Casey intends to find out. George has to prevent himself from getting the chicken pox Derek has, as he has a job presentation.

The McDonalds and the Venturis kids (Casey and Lizzie vs Derek, Edwin, Marti) decide that the best places in the house aren't big enough for the two families, so they agree to decide by games of chance (dice, cards, etc).

Derek sabotages Marti when he babysits her in an attempt to impress a girl.

After a clumsy mishap, Casey sprains her ankle and blackmails 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 blackmail her. Both turn out to be in vain when Marti decides to make a "picture" with the tickets.

Casey notices Derek, Edwin, and Sam playing a sexist video game (Babe Raider) and immediately launches a campaign against the sexist attitudes in the house.

Chaos strikes the household. Derek dyes the white clothes pink, uses the neighbor's pool and stomps on their flowers, Marti makes "potions" with Casey's perfume and shampoo and hits the neighbor's son. Nora schedules a book club at the house, which she forgets to tell George about, and the family tries some last minute cleaning to look organized.

Casey likes Derek's best friend, Sam, who has feelings for Casey too. Derek is opposed to a relationship between his best friend and his stepsister so he tries to sabotage the possibility of them coming together by informing Sam of the "Male Code", which says a guy's best friends cannot date their friends siblings (or step-siblings). Derek tries to break Casey and Sam apart but fails to do so.