For a 13-year-old, Antoine is still quite childlike and dreamy. He spends the summer holidays with his family by the sea, as he does every year, when unexpectedly, visitors arrive: a friend of his mother and her daughter Hélène. Hélène is 16, beautiful, mysterious, and treats the boy not with the condescension of an older person, but rather takes him under her wing in a friendly manner, leading him to his first cigarette, his first high, and ultimately his first kiss. Hélène awakens his sexual desire and is simultaneously like a sister, with whom Antoine and his little brother share moments of childish camaraderie. When she says goodbye after a week, she has gently pulled Antoine out of his childhood. With "A Sister," Bastien Vivès presents a sensitive narrative that delicately captures the transition between childhood and adulthood.