At the initiative of Nath, an environmental activist and Laurent's girlfriend, the group of friends Thomas, Laurent, Marc and Aude, will embark on a punitive expedition to a cosmetics laboratory. A series of incidents will cause an inexplicable phenomenon, a lightning bolt that strikes Thomas and causes a strange burn on his arm. And the next day, Thomas, stunned, wakes up transformed into a girl... Justin, his little brother, and Aude, his best friend, will be in on the secret and help Thomas hide this strange phenomenon.

Thomas, very disturbed by what happened to him, realizes that his transformation can be caused by a shock to his scar. He dreads going back to school more than anything, and will try to escape it by pretending to be sick, which is also contagious. Aude accompanies him to the doctor, but Thomas runs away and prefers to continue to hide everything, even if he gets a little tangled up in all his lies.

Finally back at high school, Thomas falls head over heels in love with Ally, a senior. His friends will overwhelm him with good advice on how to seduce her. So he will try his best to meet her, but he must also learn with Aude to master his new girly appearance, the all-important problems of clothes and makeup. And apparently, his "girl" version is more successful than his "boy" version. But with Ally, it's not a given this time.

Determined to make some pocket money, Thomas is forced to transform himself into a girl to get a babysitting job. He will discover that it is not so easy to look after a little pest named Lola, and to manage a baby's diapers. Fortunately, friends are there, and Aude, who had lost the diary where Thomas's whole story was told, will end up finding him where no one expected...

Faced with Thomas' disastrous school results, and the fact that they caught him dressed as a girl, his parents decide that he absolutely must see a shrink, who is also a shrink. She will not bear the revelation, and the parents will conclude that it is a classic teenage crisis.