Avril is a novice in a convent of "Baptistine" sisters, a monastic order which was officially dissolved by the end of the nineteenth century but that is kept alive by Mère Marie Joseph, the sadistic superior. While Avril is on retreat, locked in for a fortnight in a chapel prior to taking her vows, Soeur Bernadette, a sympathetic sister, discloses a secret to her: she has a twin brother.