A robotics engineer at a toy company creates a highly advanced doll, M3GAN, that becomes sentient. As Cady, the orphaned girl under her guardianship, forms a strong emotional bond with M3GAN, the doll's behavior becomes increasingly sinister and violent. Gemma, Cady's aunt, must unravel the mystery behind M3GAN's actions and protect herself and Cady from the doll's malevolent intentions.

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.