A brilliant toy company roboticist uses artificial intelligence to develop M3GAN, a life-like doll programmed to emotionally bond with her newly orphaned niece. But when the doll's programming works too well, she becomes overprotective of her new friend with terrifying results.

After the underlying tech for M3GAN is stolen and misused by a powerful defense contractor to create a military-grade weapon known as Amelia, M3GAN's creator Gemma realizes that the only option is to resurrect M3GAN and give her a few upgrades, making her faster, stronger, and more lethal.