The plot of the movie concerns Kelly, a free-spirited eighth grade girl whose mother gets married to Brigadier General Joe "Sir" Maxwell. When her new stepfather becomes the Commandant of a military school, George Washington Military Academy, Kelly and her family move upstate. Kelly has to enroll at the school, since it is the only school in the area, leaving behind her art school and her best friend, Amanda. On her first day at military school, she befriends Carla, a girl who has been there for a long time and shows her the ropes. Kelly, at first, has trouble fitting in and obeying the orders of the officers above her, especially Cadet Captain Jennifer Stone, who has a crush on Cadet Major Brad Rigby. Kelly feels drawn to Brad instantly, and competes with Captain Stone for his attention. Captain Stone does not treat Kelly well, verbally abusing her and destroying her personal belongings. To get back at her, Kelly decides to paint Stone's hair in the middle of the night, in the same pattern as a blanket of Kelly's that Stone ruined. Kelly is ready to tell her mom about her feelings and opinions of the school, when her mother reveals that she's pregnant. Seeing that her mother needs her support, she vows to help her stepfather learn how to take care of a child and make him "ready" to be a dad.
After Kelly accidentally ruins Stone's dress at the school dance, she is forced to appear in Cadet Court and is found guilty of many infractions. She is sentenced by her stepfather to take care of and shine the uniforms of the drill team, which she had, earlier in the film, referred to as a team of robots. However, she gradually takes a liking to them and decides to try out for the team, seeing that they need some inspiration. With the help of Gloria' a fellow cadet, she practices enough and makes the team.
Kelly's team makes it to the regionals, which will be held at a different school. Kelly's dad surprises her by revealing that he will be working a job nearby, and can make it to the regionals to see her perform. The day of the competition, however, he does not show up and Kelly begins to worry. Her stepfather notices that she is distressed and asks her to explain the situation to him. Kelly tells him that she has received a phone call from her father that was cut off, which she finds strange. She refers to her cell phone as her and her father's "lifeline", and says that he would only call if it was an absolute emergency. Seeing that Kelly needs help, he excuses her from the competition and goes with her to the location her father said he would be. They find him on a cliff, as he has fallen down. Kelly doesn't want to leave him alone, so she uses her new training to rappel down the cliffside and stay by her father. Joe calls for help, and a rescue team arrives to bring him and Kelly back up. Realizing that Joe has become more fatherly, she tells both of her dads that she is proud to be their daughter.
Kelly runs back to the competition to find that they are down by five points. The only chance they have of winning is a special routine that Kelly and Stone had been practicing. They perform it, and receive excellent marks from the reviewing board. George Washington's drill team places second, but only by a difference of one point. Kelly gives Stone a hug, which she is surprised by, but returns anyway. She also makes amends with Carla, who she had offended by saying that Amanda was her truest and best friend, even though Carla had always been there for her. The drill team vows that they will win next year, but Stone reveals that her father, a military man, has been transferred to Europe and she will be leaving with him. Stone remarks that she would like to see Kelly take her place as a cadet officer one day, and among her responsibilities have to put up with new recruits like herself.
- Hilary Duff as Kelly Collins
- Christy Carlson Romano as Cadet Captain Jennifer Stone
- Gary Cole as General Joe "Sir" Maxwell
- Andrea Lewis as Carla
- Shawn Ashmore as Cadet Major Brad Rigby
- Aimee Garcia as Gloria
- Sarah Gadon as Amanda
- Linda Kash as Samantha
- Nigel Hamer as Adam
- Avery Saltzman as Kevin
- Joe Matheson as General Archer
- Beverlee Buzon as Grace
- Dalene Irvine as Marla
- Desmond Campbell as Lt. Col. Ross
- Tim Post as Col. Mikkelson
- Christopher Tai as Sr. Cadet officer
- Josh Wittig as Cadet bugler
- Edie Inksetter as Math teacher
- Stewart Arnott as Captain Lawrence
- Lt. William T. Bates Military Advisor
- Martin Roach as Drill instructor
- Ashley Leggat as Dancer (uncredited)
- Steve Bryan as Background Performer (uncredited)
Primary filming for the school scenes was done at the Robert Land Academy, a military school in Canada. There were also some scenes that were taken in St. Andrew's College, a private school and Loretto Abbey Catholic Secondary School, also in Ontario, Canada. Hilary Duff was made an honorary Cadet Sergeant of the school during the production. The military advisor for the film, Lt. William T. Bates, drilled the actors in a "boot camp" style situation, often testing them with rehearsals of complicated rifle and close order drill routines. The final scene was shot at the Canadian Forces Fort York Armoury in Toronto, Ontario.