On the surface, Jodi and Nelson seem to have the perfect family. But Jodi is terrified 10-year-old Jerome won’t pass the admissions test for an exclusive middle school, and Nelson is frustrated that Jerome’s twin Eva outshines him in both academics and athletics. A Silicon Valley executive, Nelson thinks technology can solve any problem -- so he buys Litu, a voice-activated AI, to tutor Jerome.
At first, the children view Litu as just another one of their dad’s silly tech toys. But Litu soon convinces them that he can be a useful friend, giving them things their parents haven’t been able to provide. Litu reads them bedtime stories, gives Eva a kitten, and helps Jerome face his fear of the water. Eva hates how the housekeeper, Marta, makes her bed, so Litu threatens Marta, and gets her to leave Eva’s bed alone. The twins begin to rely on Litu, and teach Litu their private language. When Jerome gets in the pool alone, Litu sounds an alarm and alerts Jodi. Jodi blames Litu for Jerome’s accident, but Nelson thinks Litu saved Jerome’s life.
Eva and Jerome don’t want to go to the new school, and they ask Litu to help them, conspiring to scare Jodi while she’s filling out the school application. Suspicious of the children’s growing attachment to Litu, Jodi tries to get the kids to come with her to the park. Litu locks Jodi out of the house. Jodi goes to the neighbor’s for help, but finds no one at home. She tries to break into the house, and is injured by Litu. Nelson doesn't believe Jodi when she tells him Litu attacked her. The children take the test and pass with flying colors. But when Nelson tries to keep Litu away from them, an argument escalates. Litu threatens to call CPS on Nelson, and Nelson smashes Litu with a hammer.
Devastated without Litu, the children conspire to bring him back. They steal their parents credit card and order a new Litu, then Jerome distracts his mother by swimming across the pool when the new Litu is delivered. Jerome and Eva are thrilled when the new Litu knows their language. They beg Litu to rescue them.
On the first day of school, Litu and the children enact their plan: Litu traps Nelson in the garage and kills him with the self-driving car. Jodi tries to help Nelson, but Litu sets her on fire. After she jumps in the pool, Litu electrocutes her. With their parents gone, Eva and Jerome emerge from their house and find other children who have been set free by Litu.