Brilliant toy-company roboticist Gemma (Allison Williams) designs M3GAN, a lifelike artificial intelligence doll programmed to be a child's greatest companion and a parent's greatest ally. When her eight-year-old niece, Cady (Violet McGraw), is orphaned, Gemma becomes her guardian.

Unsure of how to suddenly become a parent, Gemma decides to pair her M3GAN prototype with Cady in an attempt to resolve both problems — a decision that will have unexpected consequences when the robot becomes violently overprotective of her new friend.