In a toy company, a robotics engineer has built a life-like doll that begins to take on a life of its own. The CEO of the American toy company calls it the biggest technological innovation since the invention of the car. He is referring to M3GAN (pronounced Megan), the robot doll developed to serve as a friend and teacher, playmate and protector.

Gemma, who develops the lifelike doll for the company, decides to test her invention on her eight-year-old niece Cady. Gemma is temporarily appointed as guardian of the orphaned and traumatized girl. And lo and behold, the robot doll turns out to have a healing effect. M3GAN can talk and sing. She plays with Cady, makes jokes, and takes on educational tasks in between.