Viola Hastings is a teen girl who plays for the girls soccer team at her high school, Cornwall Prep. Her dream is to play for the North Carolina Tar Heels. However, the team gets cut. Viola and her friends try to join the boys' team, but the coach refuses. Viola's boyfriend, Justin, supports the coach's decision, upsetting Viola, and resulting in their breaking up.
Meanwhile, her twin brother, Sebastian has to enroll in Illyria, an elite boarding school, as he was recently expelled from Cornwall for skipping classes, but he secretly goes to London with his fledgling band instead. Sebastian is also struggling with his sex life and relationship with his shallow girlfriend, Monique.
Viola agrees to cover for Sebastian by telling each of their divorced parents that he is staying at the other's house. Viola decides to pass herself off as Sebastian, hoping to join their boys' team and beat Cornwall to prove their coach and her cocky ex-boyfriend, Justin, wrong for suggesting cancellation of the ladies' soccer team. With the help of her stylist friend, Paul, she is transformed into "Sebastian" and attends Illyria in his place.
While moving in, she meets her roommate, Duke Orsino, Illyria's attractive soccer team captain. During tryouts, Viola fails to impress Coach Dinklage and is assigned to second string, much to her dismay. Her teammates, including Duke, initially dislike "Sebastian" as he's awkward and strange. However, with help from Paul once again, they begin to accept him into their social circle.
"Sebastian" then gets the popular and beautiful Olivia Lennox as his lab partner, which frustrates Duke, as he has feelings for her. "Sebastian" agrees to put in a good word for him if he trains him to be a better soccer player. Coach Dinklage eventually notices "Sebastian's" effort and improvement, promoting him to first string.
At the Junior League carnival, where her mother has made her volunteer, Viola works a shift at the kissing booth and kisses Duke. Duke expresses to "Sebastian" that he might move on from Olivia as he is starting to like Viola now. She is delighted as she secretly feels the same way.
Olivia, who now has a crush on "Sebastian", asks Duke out on a date, hoping to make "Sebastian" jealous. Viola, who is unaware of Olivia's true intentions, is enraged instead because Duke has now abandoned his interest in Viola. When she finds out the truth, she encourages Olivia to tell "Sebastian" directly how she feels.
The situation becomes complicated when the real Sebastian returns from London a day early, unbeknownst to Viola. As soon as he arrives at Illyria, Olivia confesses her feelings and kisses him. Duke, seeing this, believes his roommate has betrayed him. When "Sebastian" returns to their room, they have an argument and Duke kicks him out. Viola oversleeps and misses the first half of the game, while the real Sebastian is mistaken for "Sebastian" and winds up poorly playing his sister's game instead. At half-time, Viola explains the situation to him and they switch places again.
Duke, still furious at "Sebastian", refuses to cooperate with him on the field. Determined to make amends, "Sebastian" shows everyone he is actually Viola by flashing her breasts. Illyria wins the game when Viola scores a goal, finally humiliating Justin and the rest of the Cornwall boys.
Everyone at Illyria celebrates their victory over Cornwall, except for Duke who is hurt about Viola's deception. Viola introduces Sebastian and Olivia officially, and they begin dating. She and Sebastian's divorced parents also make up, exchanging contact information to be better parents for their children. She invites Duke to her debutante ball, through an invitation delivered by Sebastian, now Duke's actual roommate. Still hurt, Duke doesn't respond to the invitation, devastating her.
At the ball, Viola fears Duke won't show up; she distracts herself by assisting Olivia, who is being escorted by Sebastian to the ball, and is touched that Paul offers to be her date. Her mother shows up with a dress suiting Viola's "no ruffles" policy, but Viola decides to go for a walk instead.
Viola finds Duke outside, who admits he has feelings for her, but insists on no more deception, which she promises. At the ball, Monique is escorted by Justin, Olivia by Sebastian, and Viola and Duke enter the stage late, but together, with Viola in her new dress, much to the joy of her mother. Viola and Duke share a kiss before joining the crowd. At the end of the film, Viola and Duke are shown happily playing on Illyria's soccer team together.
