After the death of his grandmother Emma, Robin and his mother Claudie find a collection of letters sent to Emma by a girl named Marcelle. Marcelle and Emma met and secretly loved each other in the 1920s during they adolescence, before their ways separated. From the sanatorium where she was hospitalized to treat tuberculosis, Marcelle wrote Emma letters that still burn with passion. At the sanatorium, rebellious Marcelle, nicknamed ‘Ultraviolette’, led a group of three young sick women. Through Marcelle’s suggestive letters, the film breaks down the barriers of time combining archive footages, avant-garde films, and music to create a sensuous, poetic atmosphere of absolute love, the profile of a courageous woman, ahead of her time.