When robotics engineer Gemma becomes the guardian of her orphaned niece, Cady, she thinks her new invention, a robotic AI, will be a good companion. However, M3GAN begins to behave in unexpected and shocking ways.

Two years after M3GAN's rampage, her creator, Gemma, resorts to resurrecting her infamous creation in order to take down Amelia, the military-grade weapon who was built by a defense contractor who stole M3GAN's underlying tech.