In a secluded mountain retreat, a psychology graduate named Simone invites her old friend Rachel to join her in a self-made research project exploring identity, memory, and regression. Rachel’s 19-year-old cousin Tyler, a quiet, sensitive young man, tags along. Simone has been preparing for years to test her advanced hypnosis-based regression therapy and sees in Tyler the perfect subject to become "Tilly," the sweet, docile baby girl she’s always dreamed of nurturing.