Set in year 1959, the film opens with an introduction to the quaint little town of Suburbicon. Established in 1947, it has become a generally pleasant town with people from other parts of the country moving there for an idyllic lifestyle.
Mailman Henry (Steve Monroe) goes around happily greeting the town's residents. He reaches the home of the new residents, the Mayers family, who happen to be African-American. When Henry speaks to Mrs. Mayers (Kamirah Westbrook), he doesn't realize she is the new neighbor until she says so. His friendly demeanor drops as he awkwardly tries to step away while "forgetting" to give Mrs. Mayers her mail. The other residents then react with horror to the fact that their new neighbors are black.
A town meeting is held full of angry and racist white men demanding that the Mayers be removed from the town. A petition is held among all the all-white townsfolk to do something about getting the Mayers family to move out.
The paraplegic Rose Lodge and her twin sister Margaret (both played by Julianne Moore) are the Mayers' next door neighbors. Rose tells her 10-year-old son Nicky (Noah Jupe) to play with the Mayers' son Andrew (Tony Espinosa). Reluctantly, Nicky goes over and invites Andrew to play catch with him.
That night, Nicky's father Gardner (Matt Damon) goes into his room with another man, Louis (Alex Hassell). Gardner tells Nicky that Louis and another man, Ira Sloan (Glenn Fleshler) are about to rob them. Gardner takes Nicky downstairs where Sloan is sitting in Rose's wheelchair, and she and Margaret are being kept hostage. After they force Gardner to pour them drinks, the robbers sit the family at the dinner table and smother them with chloroform rags.
Nicky wakes up in the hospital to find Gardner and Margaret, and to see that Rose is dying after being overdosed on chloroform.
We cut to Rose's funeral. Rose and Margaret's brother Mitch (Gary Basaraba) comforts Nicky and gives him some extra money.
People around town offer their condolences to Gardner, Margaret, and Nicky everywhere they go, while quietly insinuating the Mayers family may have been involved.
Nicky starts to take precautions to avoid another incident. He sets up bars against his bedroom door to prevent anyone from breaking in. He hears what sounds like Margaret yelling in fright. Nicky grabs a knife and goes to the basement, only to find that Gardner and Margaret are engaged in kinky sexual activity, with Margaret having cut her hair and dyed it blonde to resemble Rose.
Gardner is contacted by Captain Hightower (Jack Conley) and asked to come to the police station to view a lineup of suspects. Margaret arrives at the station and brings Nicky because she didn't want to leave him home alone. Nicky is left outside the room as Gardner and Margaret go in. Nicky sneaks in as Gardner is attempting to identify the men. Nicky clearly spots Louis and Sloan as the last two in the lineup. A guard mistakenly flips the light switch on, revealing the family behind the glass and making Gardner and Margaret see that Nicky made it inside.
At home during a bath, Nicky asks Gardner why he didn't identify Louis and Sloan when he had the chance. Gardner dodges the question and says it's just complicated.
All around town, the racist residents of Suburbicon continue to show increased hostility toward the Mayers family, particularly at the supermarket where Margaret works where the manager tells Mrs. Mayers that everything she has taken has been raised to $20.
Louis and Sloan confront Gardner at work in his office. Sloan smacks him around and is angry that Nicky had seen them, and is also demanding money that was supposed to be returned after Rose was killed.
Hightower visits Gardner at work to inform him that an enforcer for the mob named Frank Rizzoli had been found dead. After going through his books, it was discovered that Gardner's name was listed among those that owe money to the mob. However, Hightower also notes that they did find a hotel called Garden Lodge, and decides perhaps they were confusing it with Gardner's own name.
One afternoon, Margaret is visited by a sleazy insurance salesman named Bud Cooper (Oscar Isaac). He smooth-talks Margaret before making clear his true intentions after he mentions that an insurance claim was filed by Gardner shortly after Rose's death. Margaret lets it slip that she and Gardner were in need of money after Gardner had attempted to start up his own business and subsequently failed. Margaret kicks Cooper out of the house as he continues to press her.
Sloan approaches Louis at work and tells them that they need to take care of Nicky since he is going to get suspicious of the whole scheme. Sloan orders Louis to finish the job, or he will become a liability.
Throughout the day, the town's residents gather outside the Mayers' home to taunt them before it gradually turns into a riot with police forced to take serious action. The mob breaks all the windows of their home and set their car on fire while planting a confederate flag. Meanwhile, Cooper visits the Lodge house. He tells Gardner he knows about their insurance fraud and that he is going to blackmail Gardner and Margaret with bringing the police over unless he receives the whole cut of the insurance claim. Margaret brings Cooper a cup of coffee, and he begins to choke after drinking it. Margaret admits to Gardner that she put lye in it. Cooper runs out in a panic as Gardner grabs a fire poker. He chases Cooper outside and swings the poker into his head, killing him. Louis and Sloan are nearby watching. Gardner stuffs Cooper into the trunk of his car and drives away to dispose of him, while Louis goes inside the house and Sloan follows Gardner.
Nicky realizes what's going on and he calls Mitch for help. Margaret cuts the phone line, and Nicky locks himself in his room. Margaret makes a peanut butter and jelly sandwich that she laces with crushed up pills to poison Nicky, as well as his milk. Nicky refuses to touch the food. Louis sneaks up on Margaret and strangles her to death before making his way up to Nicky's room. He finds Nicky under the bed but is pulled out by someone else. After a brief struggle, the other person shoots Louis dead. It turns out to be Mitch. He gets Nicky out and brings him to hide in a closet. Mitch turns around to reveal a knife in his back from the fight with Louis. He sits down by Nicky's bed and slowly dies.
Gardner takes Cooper's body out of town and leaves it in his car. He rides a bicycle home and is followed by Sloan. While distracted and yelling at Gardner, Sloan is struck head-on by a firetruck. The leaked fuel catches fire and causes the car to explode, killing Sloan. Gardner rides home.
Gardner finds Nicky in the closet with a gun. He also finds the bodies of Margaret, Mitch, and Louis. Gardner sits at the kitchen table with Nicky as he starts to eat the poisoned sandwich and drink the milk. He calmly tells Nicky that he can join him in moving to Aruba with the money he will take out from the insurance claim, or Gardner will kill him.
The following morning, the Mayers family cleans up the mess around their house. The unrepentant and racist neighbors continue to taunt and use them as scapegoats for the chaos that has occurred in town. Nicky is watching the news on the TV set about the race riot while Gardner lies dead at the table. Nicky goes outside and sees Andrew. He takes a ball and starts playing catch with him.