The opening scene depicts the parasitic life-cycle of cuckoos before introducing Gemma (Imogen Poots), a primary school teacher, and her boyfriend Tom (Jesse Eisenberg), a handyman. They drive to meet with a real estate agent to inquire about buying a house. The agent, Martin (Jonathan Aris), is oddly off-putting. Nonetheless, they follow him to a suburban development called Yonder, filled with identical houses. There, Martin shows them around house number 9. He asks if they have children, and when Gemma replies, "No, not yet" he mimics her perfectly. After looking around the garden, they re-enter the house to find that Martin has vanished. They attempt to leave, but repeatedly find themselves returned to number 9. In disbelief, they drive around the endless, identical streets until they run out of gas. There is no phone signal either.Out of options, they sleep in the house. The next morning, they try to escape on foot, but consistently return to number 9. They eventually give up and find a box full of tasteless vacuum-packed food left in front of the house. In desperation, Tom sets fire to the house, and they sleep on the pavement. When they awake, another box has been left for them. This time it contains an infant, and the message, "Raise the child and be released." When the smoke clears, they see the house undamaged.98 days later, the infant has grown to resemble a ten-year-old boy who, like Martin, mimics Tom and Gemma. It screams bloody murder when it gets hungry and stops when it is fed. Gemma and Tom sit together in the garden with a pickax, waiting to attack whoever delivers and collects the food, but they never see anyone. Tom starts digging a hole in the garden, thinking it may lead somewhere, and becomes withdrawn. The boy secretly watches them have sex at night. In the living room, the Boy (Senan Jennings) sits before the television, which shows bizarre, fractal-like patterns.Tom comes to believe if the Boy dies, someone might come for the body, and they could force whoever it is to free them, so he locks it in their car, intending to starve it. However, Gemma takes pity and releases it. One day the Boy goes missing and returns with a book full of strange symbols and images of humanoids with throat sacs. When Gemma asks him to mimic the person who gave him the book, he makes strange, rasping sounds and inflates his throat sacs. Gemma finally had proof that the boy is a different species, possibly alien, who use humans to raise them.Soon the Boy has matured to resemble a young adult (Eanna Hardwicke). Tom and Gemma avoid him where possible as Tom becomes sick. The Boy leaves during the day, and Gemma tries to follow him, but always finds herself back at number 9. Tom continues to dig and finds a body in a vacuum bag. The Boy locks Gemma and Tom out of the house, forcing them to sleep in the car. When Tom's condition worsens, Gemma pleads with the Boy for medicine for Tom, but he replies, "Maybe it's time he is released." When Tom dies in Gemma's arms, the Boy zips him into a vacuum bag and throws it into the deep hole Tom has dug.After another night locked out, Gemma ambushes the Boy with the pickax, but only manages to wound him. The creature hisses and crawls into a labyrinth under the pavement. Gemma follows it and crashes through the door into multiple rooms in other houses with more Boys and several strangers, one of whom has committed suicide. She lands back in number 9, weak and moaning. The Boy is cleaning the house. He carries her to a vacuum bag and explains that mothers die after raising their sons. She says she is not his mother, and dies as he zips her in. The Boy buries her with Tom and fills the hole, which then disappears. After filling the car with gas, he drives back to the estate office, where an aged Martin lies dying in his chair. Martin gives his name-tag to The Boy and then expires, having finished his life-cycle. The Boy puts Martin in a vacuum bag, folds it up, and puts it into a file drawer. He sits down in the chair and waits. When another couple walks in the door, the Boy stands to greet them the same way Martin did.