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.
When Derek asks Casey's best friend, Emily, out for a date, Casey has to wonder: Is he really interested in Emily, or is this some kind of revenge for "stealing" Sam?
Derek and Casey have a competition with who is the best sister or brother. However Lizzie has a problem playing hockey and Derek is the only one who could be her trainer and seeing as Derek is always with Lizzie, Edwin asks Casey for help about girls. What could possibly go wrong?
Derek couldn't care less that Casey and Sam are always breaking up and getting back together, until they break up for good and Sam's ability to play hockey goes down the tubes.
Casey has some great ideas to improve the school so she decides to run for school president. But, when Emily advises her to keep in mind the risk of destroying her reputation, Casey decides to enlist Derek to run for her, since he is popular.
Derek's band is so desperate for a singer that they ask Casey to join; even though she's more folk than metal and thinks they're a bunch of posers. George, remembering his rock 'n roll youth, longs to join the band himself.
Casey, compromising all her principles, takes Derek's advice and lies when she desperately needs an extension on a school assignment. Soon she is trapped in a web of deceit and the harder she tries to extricate herself, the worse it gets.
When Casey becomes attracted to Scott, Lizzie's soccer coach, he asks her out. But Lizzie too, is attracted to Scott. Derek feels guilty after hearing from Scott that he's dating other soccer sisters. Meanwhile, George is upset after Nora says that she did not feel love at first sight with George.
To pay Derek back for his many practical jokes, Casey pulls a huge prank of her own: stealing the office furniture from the new uptight Vice Principal. Everyone assumes Derek did it. The only two people who know he didn't are Casey and Derek.
While Casey is freaking out about her upcoming dance competition and Derek is freaking out about his upcoming hockey playoffs, Lizzie and Edwin decide they're sick of being the ever-accommodating middle-kids.
Derek keeps making bets with people and when Casey wants to join in, she bets that she can get a punk guy, Trevor, from school to ask her out. George and Nora worry about Casey's new look because it scares their family. Meanwhile, Edwin bets Lizzie that someone will call him cool.
When Derek starts to have a crush on a girl named Kendra (Casey's new study partner), Casey tricks him into changing his laid-back image into a more mature person because he wiped out Casey's report on the computer. While George, Edwin, and Derek are afraid of the mice living in their house they try to kill them, but Lizzie won't allow it and makes the mice her new pets.
Casey tries to get rid of Derek for the dinner party she has planned with her biological father. But to Casey's surprise, Derek and the rest of the Venturis get along great with her father, leaving her jealous.
Kendra volunteers to help Casey get a boyfriend by throwing a small get together at the Venturi-McDonald home. However, Derek uses the opportunity to throw a huge house party.
When Derek already has a girlfriend, Casey is shocked when he asks out a friend of hers, Lucy. Casey asks Emily to dig up information on whether Max is available or not. Meanwhile, Nora has set up a dinner with another family, but some members of her family object.
George and Nora plan a joint birthday party for Lizzie and Edwin. It's a fiasco until Casey and Derek reluctantly provide emergency help.
Casey and Derek turn out to be the world's worst babysitters, so Edwin invites his friend, troublemaker Teddy, over to stir up some trouble, such as tying Derek to his chair, tickling his feet, and forcing him to eat broccoli. Casey sees Max, with his ex-girlfriend Amy and assumes they are getting back together.
Nora decides to quit her job for a while to spend more time with her family who she assumes she's been neglecting. But Casey risks losing her 'cool' status when Nora tries to recruit her new friends to help at the high school fun fair.
Casey is shocked when Derek is selected to be a prefect at school and sets out to prove he's not the man for the job; Edwin and Lizzie are jealous that Marti always gets her way.
Casey and Derek both land jobs at the same restaurant, and things don't go quite as Casey expected when Derek is a great waiter and she is a terrible waitress.
Edwin and Derek both battle school bullies, and when Casey finds out about the one who's threatening Derek, she does everything she can to thwart the potential showdown.
Casey tries to film a heartwarming saga of a blended family for her school project, but her family members won't cooperate.
When Derek gets bumped up from the chorus into a lead role in the school musical, Casey gives him some advice so he won't ruin the show.
Derek is forced to attend summer school, but sneaks out and injects some fun into Casey's dull camp curriculum; Edwin and Lizzie set up a lucrative pet-sitting business.
Derek switches test papers with Casey, and then lures her into a scheme that will keep them both out of trouble; Edwin, Lizzie, and Marti feel that they are doing too many chores for the amount of allowance they are getting.
George and Derek's mother Abby are dismayed Derek is failing Spanish, and decide that he should live with Abby in Spain for six months.
When Casey lectures Derek and Edwin for watching scary movies—which she finds silly and fake—they take her admonishment as a dare to scare.
The family stages an intervention to convince George to buy a new car; Derek faces the scary possibility of spending his 16th birthday at home with his family.
Derek swears off girls until he meets Sally, the attractive new waitress at Smelly Nellie's. Lizzie is freaked out when her best friend Jamie tells her he likes her.
When Christmas plans go awry, Aunt Madge encourages Derek to help restore the family's holiday spirit.
After being pranked by Derek, Casey wishes Max could be chivalrous like the literary hero Ivanhoe.
After Derek breaks up with Kendra so he won't have to buy her a Valentine's Day gift, Kendra asks Casey's boyfriend Max to the Valentine's Day dance. George forgets to plan for his first Valentine's Day with Nora.
When signals get crossed, Derek thinks he has a chance with Sally, and Casey is sure Max is going to dump her.
Casey's 16th birthday is coming up, and she is planning a sophisticated dinner with her friends, but Derek has other ideas, and all plans go horribly wrong with an unexpected trip to hospital.
Casey is thrilled to get her driver's license before Derek, until she gets stuck playing chauffeur to her younger siblings. Meanwhile, Derek fails his driving test again, and George gets his license suspended.
Casey's dream of a romantic prom night turns into a nightmare when Derek accidentally ruins her dress.
Casey tries out for the school cheerleading squad. But not even her phenomenal dancing skills are enough to get her acceptance, especially when the head cheerleader is Max's ex-girlfriend.
Casey develops a mysterious allergic reaction after Max gives her his football jacket. George gets fed up with Derek's jokes and makes a deal with Derek that if he can be nice for one week, he won't have to pay for the insults he said.
Grounded for the weekend for fighting again, Casey and Derek try to sneak out to go to a party, but they end up putting a dent on George's car.
Derek and Edwin both strike out despite Derek's patented dating techniques.
Derek refuses to help out with Marti's birthday party despite being asked by Sally. Casey wants to get back together with Max, but he is hesitant about the whole idea.
Derek has trouble expressing his feelings, so Sally suggests that he write her a song. Edwin is hurt when no one finds his comedy routine for the talent show funny.
Derek becomes jealous when Sally and her ex-boyfriend Patrick start spending more time together. Derek claims that people of the opposite gender cannot be friends with each other, and Casey sets out to prove him wrong. Edwin is suspicious that George and a close friend of his are having an extramarital affair.
While the rest of the family heads off to Quebec for spring break, Casey and Derek decide to stay home. However, the two discover that they now have to spend the entire vacation together when their original plans don't work out.
Casey is crushed when a new kid at school named Truman starts rating girls, and only gives her a 6½ out of 10.
Derek breaks up with Sally after she announces her plans to attend an out-of-province university in the fall. Derek claims he's fine with the breakup, but Casey and Edwin don't believe him. Meanwhile, Lizzie and Marti take care of a lost dog.
All the kids are in crisis about going back to school in September, but they have to put their problems aside to stop Derek from dropping out of school to go west with Sally.
Casey is in panic mode after the diary that details her unsettling romantic dreams about Truman goes missing. Believing that Derek and Edwin took it, Lizzie and Marti attempt to get the diary back by making their stepbrothers' lives miserable.
Casey spends the day with George at his law office and takes the initiative to reorganize his filing system, with disastrous results. Derek tries to help Nora land a business deal with a wealthy client.
Casey gets stuck with Truman as her fencing partner in gym. What starts out as a friendly game of table-tennis between Derek and Edwin turns into an ongoing competition to figure out who is the best player.
As George prepares a Mexican fiesta for their second wedding anniversary, Nora gets a reluctant Casey to tell the story of how she and Derek first met.
While his parents are away for the weekend, Ralph stays at Derek's home where he begins to develop a crush on Casey.
Derek threatens to ruin Casey's dreams of winning an important dance competition; Lizzie tries to convert junk food junkies George and Edwin.
A surprise visit from Edwin's friend Teddy turns into a frequent occurrence that makes the entire McDonald-Venturi household uncomfortable. Despite warnings from Emily, Casey decides to go out on a date with Truman.
To be eligible for college athletic scholarships, "D" average Derek seeks George and Edwin's help in passing all of his final exams. Meanwhile, Nora and the girls try to get a stressed out Casey to stop over-studying and relax. When neither of these plans work, Derek and Casey are forced to help each other.
Derek serves as a chaperone for Casey who is going to a party in Toronto with Truman. Lizzie goes out on a date with a boy that the family has never met.
As the prom approaches, Casey and Derek both find themselves without a date. Meanwhile, George and Nora have something to tell the family.
A worried Casey is having difficulty writing her valedictorian speech for graduation, which she blames on Derek. Edwin is concerned that he won't be able to live up to Derek's reputation at J.S. Thompson High when he goes there in the fall.
Derek and Casey are back and continue their hilarious sibling rivalry during a family vacation to visit their grandmother at her beautiful lake front lodge.
15 years later, Derek and his "Type A" personality daughter Skyler prepare to move in with aunt Casey and her extended family.
