Becca Fisher and Tucker James are two 13 year-olds who have been best friends since birth. They live next door to each other and have shared birthday parties, babysitters and backyard swings. But now, Becca and Tucker are beginning the rite of passage known as the eighth grade. Suddenly, they are teenagers, testing the bounds of their new independence while struggling to hang on the their past.
Sibling rivalry is running rampant in the Fisher household. Becca and Ellen are running themselves ragged trying to outdo each other since the prize for the best most helpful daughter is a trip to Malta to visit their aunt. Becca rallies Tucker and Miles to help her find dirt on Ellen, while Ellen concocts her own plan. But when Becca lets one lie slip, catastrophe develops throwing her into a spiral of guilt.
Tucker and Becca suggest that their fathers let them go camping with just Christine, Miles and Horace. Convinced they'd never survive alone, their fathers decide to stay nearby in case there is a problem. When the kids set up camp, they soon discover camping is harder than they realized and begin to miss the conveniences of home. Tucker gets mad at Miles, Becca gets mad at Christine, Miles hates camping, and Christine has left Steve the dog. Horace spends all his time fearing the dreaded ""Laughing Phantom"". However, in the middle of this fiasco, Tucker gets his sense of humour back. He convinces them all to band together to get the campsite in order, just in time to show their fathers they have it under control after all.
Becca wants to invite Rob Wetzel to a school dance, but she doesn't know if he likes her. So Kerry suggests that they create a ""Cool Book"" and pass it around so that others will write down what they think of everyone in school and then they'll know who's cool or not cool. Becca thinks it's a good idea at first but soon realizes that there are some things you'd rather not know. Meanwhile, Tucker agonizes over why no one has written anything about him in the cool book so he does it himself.
Parkview suddenly becomes newsworthy when a hot shot reporter named Kilroy decides to write an article on ""today's student"". This is too good to resist for Tucker and Miles and they concoct an elaborate set of ficticious behaviors to feed to the gullible reporter. Meanwhile, Becca is hoping to use this opportunity to get something of her own published and she's mortified when she sees Kilroy swallowing the bait: hook, line and sinker. After a front page ""expose"" is printed in the papers Lifestyles section, Tucker becomes the ""voice of a new generation"". While revelling in the spotlight, Tucker realizes that Becca's column was turned down, even though she was the really the one with something legitimate to say.
Tucker can't believe it when his dad surprises him with a pair of tickets to a concert that's been sold out for weeks. But before he has a chance to invite Becca, Tucker finds out that Kerry, the girl he has a crush on, would love to go. He invites Kerry instead, figuring that Becca will be none the wiser, until his father mentions the concert to her. Suddenly, Tucker has to find a way to get another ticket and it looks like his only option may be to pay a lot of money to a mysterious scalper. Without asking he borrows the money from Miles. But when Miles and Becca discover what Tucker's been up to, they're both furious and Tucker realizes that no concert is worth losing his two best friends.
Becca is torn when she realizes she'll have to choose between two very important activites when she learns that rehearsal for her annual ballet recital conflict with the tryouts for the pep squad. Should she follow her heart or pursue a surefire ticket to what Kerry assures her is guaranteed popularity? Meanwhile Tucker and Miles try to win the yearbook's annual candid photo contest. Their subject: the pep squad.
Things don't go quite as planned for Tucker and Becca on the first day of school. Now that they're finally in eighth grade, Becca is looking forward to writing for the school paper and Tucker wants to be the school announcer. However, Tucker soon learns that he's up against Jack, a smooth-talking new kid who seems determined to humiliate him. Meanwhile, Becca and her best friend Christine are disappointed to find that they aren't in the same homeroom. Becca makes an appointment for them to discuss it with the principal but when Christine is too nervous to shop up at the office, she unwittingly cause Becca to miss the sign up for the school paper.
After the shock of learning that Becca's dad once streaked when he was at school, Tucker decides to create some first-class mischief of his own. Determined to become one of the school's legendary pranksters, Tucker enlists Miles to write a computer program that will infiltrate the school's system and make fun of the teachers on all the school's computer screens. Becca, who has finally been given the chance to write a try-out article for the school newspaper, smells a story and tries to persuade Tucker and Miles to give her the scoop. The stakes rise after Miles is caught and risks being expelled.
Rumours of love begin to fly when Tucker and Becca plan to go to another school dance together. Encouraged by Max, Becca squashes the rumours by avoiding Tucker. When he asks for her help, she volunteers Christine to teach baseball to his unenthusiastic little brother Horace. But Becca's strategy backfires and she becomes alarmed when she notices how much time Tucker and Christine are spending together. She fears she's lost her two best friends and jealousy makes her jump to ridiculous conclusions.
Becca is thrilled when her old best friend Maxine has returned after living in New York since the third grade. But when Maxine shows up and announces she's now Max, Becca hardly recognizes this seemingly sophisticated person who finds Parkview really boring. Christine, Becca's current best friend, can't stand Max and Becca finds herself uncomfortably caught in the middle. Meanwhile, Tucker decides to tell Kerry how he feels about her and organizes a cosmic bowling party in the hope that he'll get a moment alone with her.
Tucker and Miles decide to try out for the soccer team and Tucker surprises everyone, even himself, with his natural talent as a goalie. Much to Jack's dismay, Tucker leads the team to victory in their first game. But when Tucker gets in trouble for grandstanding during a game, he gets kicked off the team. Meanwhile, Becca starts writing for the school paper and, in exchange for having her poetry published, is assigned to the loathed sports column. After a disastrous first attempt (she forgets to publish the scores), Becca learns the key to appreciating team sports from Tucker, who wants nothing more than to get back on the team.
Tucker is all set to play an exra in a horror movie on the weekend, until his mother reminds him that he has to spend that day looking after Horace. So Tucker tells Becca he'll take over their joint Health assignment which involves looking after a lifelike doll that has to be cared for like a real baby, if she'll look after Horace on Saturday. His parenting skills turn out to be disastrous. First he lets the doll's hand accidently fall into the toaster and then when Becca asks him to babysit another child for her in an emergency he misplaces the 3 year old child in the mall.
Because they were born on the same day, Becca and Tucker have a tradition of exchanging gifts the night before their birthdays. Becca always gets a great present for Tucker, and every year Tucker struggles to buy Becca a present that only he seems to appreciate. This year he goes to great lengths to find something she'll love, even enlisting the help of Ellen. He does find a beautiful music box for Becca but he finds himself stood up when Becca doesn't show. Becca was at the pizza parlour pining over her latest crush, Gooch. Tucker feels betrayed and a guilt-ridden Becca decides to make this a birthday Tucker will never forget. She succeeds with the help of Ellen and the assistant of TV personality Mike Miller, who happens to be Tucker's favourite comedian.
When Max makes fun of Becca and Christine for not having social lives, the girls decide something has to be done. Becca gets up the nerve to call a guy named Scott she knew at camp, and invites him to a movie that weekend along with a friend for Christine. Becca is both thrilled and terrified that she might get her first real kiss. Unable to stifle his curiousity and jealousy, Tucker convinces Miles that they should spy on the foursome at the theatre. He discovers he knows Scott from camp as well and that he's a real jerk. Becca gets mad at Tucker when he tries to interfere but when Becca discovers Scott's true personality and her allergy to it, she needs help from Tucker to escape from the jerk and Tucker is happy to oblige.
A bully is terrorizing Tucker's younger brother Horace, so Tucker steps in to defend him, only to discover the bully is actually his arch-enemy Jack's younger cousin. Jack then challenges Tucker and Horace to a fight against Garth and him. Meanwhile, Becca and Christine make friends with a stray dog that has been disrupting classes and try to save the mutt from the dogcatcher.
Having won the speech contest for the past two years, Becca is determined to hold on to her title this year. After carefully choosing rock climbing as her topic, she throws herself into her research only to find that her academic rival may Vega may also have the same idea. Forced to go to the extreme-and terrifying-measure of actually learning how to rock climb to get first-hand experience, Becca soon discovers that overcoming her fears is just as sweet as the thrill of victory. Meanwhile, Tucker sees and opportunity to take over the school DJ position now held by his arch enemy Jack and drafts a proposal for Mr. Petrowsky. But when Tucker loses his proposal he realizes Jack has claimed it as his own. Tucker calls on Becca to help him catch Jack redhanded and replace his as the new school DJ.
When both Tucker's and Becca's parents go away for the weekend, they decide the time is right to throw a party at Tucker's house. But Becca's sister, Ellen is planning her own party at their house and when Becca hears that the extremely cute Zach Anderson is going to be there, she forgets her plans with Tucker and their friends and begs Ellen to include her. Of course things don't go as smoothly as they all hope.
When Scott Stuckey, the town bully and Becca's former crush, shows up with his gang of skateboarders and tries to kick Tucker and Miles out of the park, Tucker takes the bait and challenges Scott to a skateoff. Becca enlists the help of Gooch, who's a skateboarding expert, but has to think fast when she accidently causes him to sprain his ankle. At first Tucker is furious and believes Becca is sabotauging him in favor of Scott, but when he learns the truth he is more determined than ever to teach Scott a lesson he won't soon forget.
Miles concocts an elaborate scheme to show Tucker that he too can be the leader in one of the escapades. He persuades Coach Petrowsky to take them on an orienteering field trip to a park not far from the Planet of the Apes film festival. Intending to sneak away to see the movies, they ditch the nature walk. After numerous catastrophes, they show up bruised and exhausted at the theatre only to discover that they've forgotten their money. Their afternoon fun, not to mention their friendship, is at stake. In the meantime, Becca and Christine have managed to lose their compass and are wandering aimlessly through the woods wondering if they'll ever reach civilization again, when suddenly their luck changes.
Christine decides that she wants to take wrestling in Phys.Ed. with the boys and Becca helps out by starting a petition to let girls enroll in the class. But when Christine shows up for practice, Tucker makes fun of her and to punish him Coach Petrowsky assigns him to the girls's rhythmic dance class. Everything comes to a head when Tucker and Becca convince a reluctant Miles and Christine to compete in a boys against girls wrestling match.
With Miles spending so much time online talking to virtual friends, Tucker feels left out. He then recruits Becca to aid him in his plan to concoct a secret admirer for Miles in order to draw him back to the real world.Becca though is supposed to be helping Christine buy a new dress for an important family function and because Christine can't keep secrets well, keeps her out of the plan. Miles however, becomes smitten with Spitfire his secret admirer and wants to meet her. Unfortunately the only thing Tucker can do is to confess to Miles and Becca must tell Christine why she keeps blowing her off. Unbeknownst to Becca and Tucker, Christine and Miles team up for a scheme of their own and give Becca and Tucker a taste of their own medicine.
When Becca finally figures out the secret location of the ninth graders's annual bash, she spills the top secret information to Kerry, who doesn't stop blabbing until everyone in the eighth grade knows. To salvage her reputation, Becca plans a new bash at her vacationing uncle's empty house. But Becca's older sister Ellen also decides to take advantage of the empty house and plans a date with her boyfriend for the same afternoon as Becca's party. Tucker, who's been grounded by his dad, sneaks out to warn Becca that her sister is on the way, but by the time he arrives, the party is already completely out of control.
Every year the eighth graders outsmart Vice Principal Petrowsky and sabotage the float for the Parade of all District Schools and this year Tucker and Christine join in. When Becca wins the Petrowsky's trust by volunteering to be Float Cop, the girls come up with a plan to sabotage the float. At this point, the guys realize it's too late to beat the girls, so instead they join them. They all sneak into the storage room and gather all the toys that the vice principal has ever confiscated from the students and they stuff the float full of them. During the pep rally, the saboteurs shoot them out of the float and into the waiting audience.
Romance blossoms as Becca stages her own adaption of ""Robin Hood"", in which she plays Maid Marion and Tucker plays Friar Tuck. The play though has been radically overhauled with Robin Hood all but written out and a romance has been concocted between Marion and Friar Tuck. Tucker and Becca both realize that they have more than friendly feelings for each other. They shyly tell each other on the phone the past times that they each crushed on the other and are surprised to learn they never liked each other at the same time...until now. On the night of the performance Tucker and Becca share their first real kiss while rehearsing backstage.
Becca's parents agree to leave her home alone if she invites a few friends to keep her company. That night, when Tucker and Miles see how much fun the girls are having, they decide to have some fun of their own. Becca, Christine and Max return home after dinner and realize they're locked out of the house. They go next door to get the spare key from Tucker's and realize the boys have something in store for them over at Becca's. Not finding the key, the girls crawl through the basement window and trouble ensues. By the end of the night, everyone has to work together to get the house back in order before Becca's parents return home.