In the 1980s, in the port city of Le Havre, Alpha is the only child of a young doctor working in a sealed hospital ward with patients infected by a virus. Thirteen years old, she is shunned by her classmates amid rumors that she carries a new disease. When she comes home from school with a tattoo of the letter "A" on her arm, her mother is devastated, fearing what illness her daughter may have caught from the tattoo needle. Her brother Amin, HIV-positive, is a hopeless drug addict with arms covered in needle marks. Alpha's tattoo begins to bleed more and more frequently, and the attacks against her at school intensify. In the school swimming pool, she nearly drowns after being assaulted by a classmate. Alpha barely knows her uncle, and when Amin comes to live with them, ravaged by illness and close to death, she truly meets him for the first time.