Antoine, 13 years old, is on holiday at the seaside with his parents and his younger brother, Titi. Both spend most of their time drawing, at restaurants and on the beach. One night, Antoine discovers another person lying in their bedroom. Hélène is 16 years old; she and her mother came to spend a few days and her presence and behavior will change Antoine's life.

In A Sister, Bastien Vivès, a master of suggestion, conveys the stumbling awakenings of adolescent desire. The story is a narrative jewel, created by one of the most important authors of the contemporary international graphic novel scene.