Mai Tokiha is a transfer student Fuuka Academy. Along a cruise ship with her brother, Takumi, the two see a person over the waters which turns out to be a girl holding a sword. A man on board the ship agrees to help with CPR on the girl and his name was Tate Yuichi. Just before that, a girl named Shiho pushes him before their lips touch. She refers to Tate as "Oneechan" (Brother) but turns out just calls him that for friendship purposes. Since no one could help the girl, Mai decides to do CPR and discovers the girl has a mark on her arm which was identical to the one on her right breast (haha...very funny...you immature people -_-). They save her and later, another woman attacks the ship and tries to attack the girl that was drowned. The girl (they don't explain her name but I'll reveal it to you: Mikoto) takes her sword and battles the girl who wants to kill her, Natsuki. Mai becomes the middle of the battle of Mikoto's blade vs Natsuki's guns as well as Natsuki's "child" (creature), Duran. The two fight and just when Duran was about to strike Mai and Mikoto, Mai cast a flame shield. Natsuki explains that she was a HiME, who were able to use special powers...
Later you find out...all HiME have a "child"...but when the "child" is killed, the person they care about most dies.