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Before their parents got married to one another, 15-year-old Derek and 14-year-old Casey ruled their respective houses as the oldest sibling. Now they're in constant rivalry for top dog status of the five kids all living under one roof. The two teens will plot anything they can think of at school, with friends or other siblings to win these funny family battles.
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.
After Derek has trouble expressing his feelings for Sally, she recommends that he write a love song for her. Meanwhile, Edwin tries out his talent show comedy routine on George and Lizzie.
Even though Sally had one friendly dinner with her ex-boyfriend, an irritated Derek decides to spend time with Kendra to make a point that girls and guys can't be just friends. Casey attempts to counter Derek's argument by searching for a male student that she can be friends with. Meanwhile, Edwin fears that George's female co-worker who is coming over for dinner is out to steal his father away from Nora.
While the rest of the family heads off to Quebec for spring break, Casey and Derek decide to stay home to meet their previously planned engagements. However, the two discover that they now have to spend the entire vacation together when their original plans don't work out.
Casey is hurt by the six and a half out of 10 that she receives from a new guy at school who starts ratings girls, and whose ratings Kendra plans to use in selecting models for an upcoming fashion show.
Derek breaks up with Sally after she reveals her plans to attend a university in another province. Believing that he is hurt over what happened and is bottling up his feelings, a concerned Edwin and Casey try to get him to open up. Meanwhile, Lizzie and Marti attempt to keep a dog hidden from most of the family.
Derek plans to drop of out of school to be with Sally in Vancouver.
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 where her need to organize things doesn't mesh well with the way he usually does things. Meanwhile, Derek tries to help Nora land a business deal with a client.
Casey gets stuck with Truman as her fencing partner in gym. Meanwhile, 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.
After entering a tv dance contest, Casey must find a new dance partner when the previous one pairs up with her rival. Meanwhile, Lizzie dares George and Edwin to eat only healthy food for a week.
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, a "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.
Derek serves as a chaperone for Casey who is going to a party in Toronto with Truman. Meanwhile, 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 worrisome Casey is having difficulty writing her valedictorian speech for graduation. Meanwhile, 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.
