Sally Acorn is a teen Mobian chipmunk girl who plays for Shark's soccer team until the team gets cut. Her dream is to play for the Shark Tar Heels. Meanwhile, her twin brother, Elias Acorn, is supposed to enroll in Tiger Boarding School, an elite boarding school, but he secretly goes to Angel Island with his fledgling band instead. Sally agrees to cover for him and decides to pass herself off as Elias, in hopes of joining their Mobian boys' team and beating Shark to prove their coach and her cocky ex-boyfriend, Geoffrey, wrong for suggesting cancellation of the Mobian girls' soccer team. With the help of her inventor friend, Tails, she is transformed into "Elias" and attends Tiger in his place.
While moving in, she meets her roommate, Sonic the Hedgehog, an attractive blue hedgehog Mobian soccer player and Tiger's team captain. During tryouts, Sally fails to impress Coach Vector and is assigned to second string, much to her dismay. Her teammates, including Sonic, initially dislike "Elias" due to his awkward and strange behavior. However, with help from Tails once again, they begin to accept him into their social circle. "Elias" then gets the popular and beautiful Megan as his lab partner, which frustrates Sonic, as he has feelings for her. "Elias" agrees to put in a good word for Sonic if he promises to train him to be a better soccer player. Coach Vector eventually notices "Elias's" effort and improvement, thus promoting him to first string.
At the Junior League carnival, where her mother has made her volunteer, Sally works a shift at the kissing booth and kisses Sonic. Sonic expresses to "Elias" that he might move on from Megan as he is starting to like Sally now. Sally is delighted as she secretly feels the same way.
Megan who now has a crush on "Elias", asks Sonic out on a date in hopes that it will make "Elias" jealous. Sally, who is unaware of Megan's true intentions, is enraged instead because Sonic has now abandoned his interest in Sally. When Sally finds out the truth, she encourages Megan to tell "Elias" directly about her feelings.
The situation becomes even more complicated when the real Elias returns from Angel Island a day early, unbeknownst to Sally. As soon as he arrives at Tiger Boarding School, Megan confesses her feelings and kisses him. Sonic, seeing this, believes his roommate has betrayed him. When "Elias" returns to their room, the two have an argument and Sonic kicks him out. Sally oversleeps and misses the first half of the game, while the real Elias is mistaken for "Elias" and winds up poorly playing his sister's game instead. At half-time, Sally explains the situation to Sebastian and they switch places again.
Sonic, still furious at "Elias", refuses to cooperate with him on the field. Determined to makes amends with Sonic, "Elias" explains that he is actually Sally. Tigers wins the game when Sally scores a goal, finally humiliating Geoffrey and the rest of the Shark boys.
Everyone at Tiger Boarding School celebrates their victory over Shark, except for Sonic who is hurt about Sally's deception. Sally introduces Elias and Megan officially, and they begin dating. Sally and Elias's divorced parents also make up, exchanging contact information so as to be better parents towards their children. She invites Sonic to her debutante ball, with an invitation delivered by Elias, now Sonic's actual roommate. Still hurt, Sonic doesn't respond to Sally's invitation, which devastates her. At the ball, Sally is skeptical that Sonic will show up; she distracts herself by assisting Megan, who is being escorted by Elias to the ball, and is touched when Tails asks to be her date. Her mother shows up with a dress that will suit Sally's "no ruffles" policy, but Sally decides to go for a walk instead. She runs into Sonic outside, who tells her that he has feelings for her, but that he doesn't want there to be any more deception on her part; Sally promises to be honest with him. Later, Hershey is escorted by Geoffrey, Megan is escorted by Elias, and Sally and Sonic enter the stage late, but together, with Sally in her new dress, much to the joy of her mother. Sally and Sonic share a kiss before joining the crowd. At the end of the film, Sally and Sonic are shown happily playing on Tiger's soccer team together
