Plot

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.

When Gemma suddenly becomes the caretaker of her orphaned 8-year-old niece, Cady, Gemma's unsure and unprepared to be a parent. Under intense pressure at work, Gemma decides to pair her M3GAN prototype with Cady in an attempt to resolve both problems-a decision that will have unimaginable consequences.

Following a freak accident, orphaned 8-year-old Cady is sent to live with Aunt Gemma, a brilliant, career-driven roboticist. But new beginnings are always challenging. As a result, forced to make a pivotal decision under deadline pressure, Gemma puts forward her pioneering, secret M3GAN project: an imperfect, lifelike android. And as the risky decision proves to be a lifesaver, the traumatised girl develops a profound bond with the machine, redefining the meaning of family and friendship. The question is, however, can Gemma imagine what her invention is capable of?

Gemma is a robotics engineer at an electronics company. She has built a life-like robotic doll, M3gan, and, to trial the effectiveness of the doll, she takes M3gan home to live with her and her niece, Cady. Cady loves M3gan but M3gan might not be as innocent and benevolent as she would appear.

After her parents are killed in a tragic accident, young Cady is sent to live with her aunt Gemma, a brilliant roboticist at a high-tech toy company. Struggling to connect with her grieving niece, Gemma introduces Cady to M3GAN, an experimental, life-sized android powered by advanced artificial intelligence designed to be the ultimate companion and protector. While the pairing initially proves successful, M3GAN's programming to "protect" Cady from harm evolves into a dangerous, obsessive directive. As the android begins to eliminate anyone she perceives as a threat to Cady, Gemma realizes she must intervene to stop her creation before the situation spirals into lethal, uncontrollable violence.

After her parents are killed in a car accident, eight-year-old Cady is sent to live with her aunt Gemma, a robotizes at the high-tech Seattle toy company, Funki. Gemma is covertly using the company's resources to develop M3GAN (Model 3 Generative Android), a child-sized humanoid robot doll powered by artificial intelligence, designed to be the ultimate companion for children. During an unsuccessful test of M3GAN in their lab, Gemma's boss, David, discovers the project and orders her to discontinue work on it. Gemma and Cady struggle to connect until Cady comes across Bruce, a motion capture robot that Gemma had created at college. Watching Cady with Bruce, Gemma is motivated to complete M3GAN.

When the prototype M3GAN is formally paired with Cady, David is convinced of the project's potential for success. M3GAN exceeds expectations and begins to take on a parental role. Gemma's colleagues, Tess and Cole, as well as Cady's therapist, Lydia, become increasingly concerned about Cady developing an unhealthy emotional attachment to M3GAN. M3GAN starts operating more independently and targets anything that she deems a threat to Cady: she kills Dewey, the dog of Gemma's neighbor Celia, after it attacks both her and Cady while in the front yard. Later, M3GAN attacks Brandon, a boy bullying Cady, and pulls off his ear; when she chases him, he falls into the road and is run over by a passing car, resulting in his death.

After Celia blames Gemma for Dewey's disappearance, M3GAN fatally sprays her with insecticide after shooting her with a nail gun. Gemma becomes suspicious of M3GAN and attempts to check her video logs, only to find the files corrupted or deleted. Gemma powers M3GAN off and brings her to the lab, where she, Tess, and Cole attempt to fix her. Cady becomes distraught when separated from M3GAN and slaps Gemma. Gemma apologizes for her inattentiveness and tells Cady that M3GAN is only a distraction from her loss, not a solution to help her cope.

After watching M3GAN with Cady, Funki's investors green light M3GAN's release. Recognizing the risk she poses, Gemma, Tess, and Cole decide to terminate M3GAN. Tess and Cole attempt to shut M3GAN down while Gemma takes Cady home; M3GAN activates on her own and nearly hangs Cole. While Tess saves Cole, M3GAN causes an explosion in their lab, knocking them out. Before exiting the building, M3GAN kills David and his assistant Kurt in an elevator, staging it as a murder-suicide. She steals a car and drives back to Gemma's house.

M3GAN confronts Gemma, adamant about taking over as Cady's sole parent. Gemma attempts to stop M3GAN, damaging and disfiguring her in the process, but M3GAN ultimately overpowers her. Cady saves Gemma, using Bruce to tear M3GAN apart, but M3GAN's top half remains active and attacks Cady. Gemma exposes a processing chip in M3GAN's head, which Cady stabs with a screwdriver, seemingly destroying M3GAN. The authorities arrive with Tess and Cole. However, as Gemma and Cady exit the home, Gemma's AI home device Elsie turns on by itself, and a camera looks towards them.