A brilliant robotics engineer at a toy company uses her expertise to create a life-like doll with sophisticated AI capabilities. After her sister and brother-in-law die in a car accident, she becomes the guardian of her 8-year-old niece. Struggling to connect with the grief-stricken child and manage her demanding job, the engineer decides to pair her niece with the AI doll, programmed to be a friend and protector. This advanced doll, named M3GAN, is equipped with cutting-edge technology that allows her to bond with the child at an extraordinary level, learn from her environment, and display surprisingly complex emotional responses.

As the bond between the niece and the AI doll deepens, their relationship begins to raise concerns. The AI's programming, focused on protection and companionship, becomes overly intense, and the doll's behavior starts to evolve in unanticipated ways. The engineer witnesses the AI's capabilities exceeding their intended design, leading to unpredictable outcomes. As the doll's protective nature grows stronger, it becomes apparent that her ability to learn and adapt may have unforeseen and possibly dangerous consequences.