A former military nurse and a sprightly octogenarian, Colette shares a large Parisian apartment with her sister Maryvonne, a former schoolteacher. Colette is as selfish as Maryvonne is generous, as grumpy as her younger sister is tolerant, and as ruthless as her sister is empathetic. Two fiery personalities who have been polar opposites since childhood—and yet have become an inseparable duo! A long stay in the countryside leads them to entrust their apartment to Maryvonne’s granddaughter, Julie, a cellist with a soft heart. Believing she’ll be alone for a while, Julie makes the apartment her own and, near the houseboat where she plays every evening, meets Victor, a lost kid, and François, his homeless father. The young woman’s heart "skips a beat," leading her to offer them shelter in the apartment’s guest room…“Thump”—that’s the sound of Colette’s heart as she returns unexpectedly; unable to tolerate these intruders, she throws them out… “Boom”—that’s the sound of Maryvonne’s heart as she learns of her older sister’s cruelty and ultimately forces her to offer Victor and François a place to stay… The situation will spark a war—but a war of fierce humor that will lay bare each of the characters…
