The 'Mommy' of the title is played by Susanne Wuest (Judas Goat, Antares), who, for a good part of the film, has her face wrapped in bandages following facial reconstruction due to an accident, or so she suggests. She is there in her large, secluded Austrian countryside home with her two nine-year-old twin boys, Lukas and Elias (the real names of the acting Schwarz twins), who aren't entirely convinced she is their real mother. Mother has been growing increasingly impatient with the boys' shenanigans, which further leads to their feeling that she's been replaced by an imposter, and they're going to get her to confess who she really is, even if it kills her.