In Belleville, Paris, Madame Rosa, Simone Signoret is an elderly French Jew and Holocaust survivor who once worked as a prostitute. She now runs a boarding home for the children of prostitutes, and among her wards is Momo, Samy Ben-Youb, an Algerian boy whom the community believes to be around 11 years old. Although Rosa is Jewish and sometimes makes racist remarks about Momo, she raises him as a Muslim in respect of his heritage, taking him to her friend Mr. Hamil Gabriel Jabbour for instruction in religion, French literature, and Arabic at the Grand Mosque. She keeps a painful secret from the world: Momo is actually 14, and she distrusts official documents and what they can prove, which makes it hard to enroll him in a regular primary school.
Momo steals a dog from a pet shop, then impulsively sells it for 500 francs and stashes the money in the sewer. Rosa, who views Momo as a troublemaker, takes him to her physician Dr. Katz Claude Dauphin under the belief that he is syphilitic or mentally ill. Later, Momo follows Rosa after she awakens from a nightmare about the Auschwitz concentration camp and discovers her hidden Jewish space under the staircase, and the two begin to grow closer.
After Momo dresses himself up as a prostitute and a real prostitute takes him to a cafe run by a friend of Rosa’s, believing that he needs help, Rosa forces him to swear never to prostitute himself or become a procurer. In a park, Momo encounters a female film editor who tells him he can visit her lab any time he likes, offering a rare glimmer of mentorship and welcome in an otherwise precarious world.
Rosa’s health deteriorates, and she begins experiencing dementia, punctuated by flashbacks to the Vel d’Hiv roundup and a recurring fear that she will be arrested by the French Police and sent back to Auschwitz. Mr. Hamil also begins to drift into dementia, finding solace in the writings of Victor Hugo. After a bad fall on the stairs, Dr. Katz informs Momo that Rosa has numerous health issues, including hypertension, and she refuses to be hospitalized. Momo contends that euthanasia could be a solution, but when Dr. Katz explains that euthanasia contradicts French values, Momo retorts that he is not French and that Algerians believe in self-determination.
Momo’s father, who spent time in an asylum after murdering Momo’s mother, returns to claim his son, but Rosa deceives him into believing that she raised Momo as a Jew, and he suffers a fatal stroke. Momo remains by Rosa’s side as she retreats to her hidden Jewish space under the staircase to die, and her body is discovered three weeks later. In the aftermath, Momo goes to live with the film editor, stepping into a new chapter that lingers with the memory of Rosa and the world they shared.
