On Father's Day weekend, architect Evan Webber remains home to complete his work project and attend his physical therapy appointment for his injured shoulder while his wife and children go on a pre-planned vacation. His wife Karen, a successful artist, leaves their assistant Louis in charge of her sculpture that needs to be moved to an art gallery for her show.

Two women, Genesis and Bel, knock on Evan's door during a rainstorm. Soaking wet, they tell him they are looking for the address of a party but their phones are not working. Evan allows them in to dry off and use the Internet. The girls play with the family dog, Monkey, and flirt with Evan while he orders an Uber for them. They disappear to the bathroom when their driver arrives. Evan brings them their clothes and finds them nude. He tries to convince them to leave, but they seduce him and he has a threesome with them.

The next morning, Evan finds that his wife's sculpture has been vandalized by the girls. When he threatens to call the police, they claim they are underage. Vivian, a friend of Karen's, stops by. Seeing Genesis, she angrily leaves. When Evan threatens to report a break-in, they agree to be taken home.

Evan returns home and cleans the mess. Genesis knocks him out with one of his wife's sculptures. Bel climbs onto him to seduce him. Evan refuses, but the girls threaten to FaceTime his wife unless he agrees to do what they demand. He is tied to his bed, and Bel forces herself on him while Genesis records, calling him a pedophile. Evan breaks free and charges at Genesis, but she stabs him with a fork; she and Bel tie him to a chair with an electrical cord.

Louis arrives for the sculpture and finds Evan; he hears the girls smashing the sculpture. He runs to stop them, but has an asthma attack and realizes they have taken his inhaler. He slips, hits his head, and dies. The giggling girls turn Louis' corpse into a red sculpture and dig a grave in the backyard for Evan. They use both Evan's and Louis' phones to send text messages in order to give the appearance that Louis discovered that Evan had an affair with Louis's wife, gotten in a fight with him, which in turn resulted in Louis's murder. The girls trash the house and torture Evan. He escapes from the house, only to be held at gunpoint by Genesis. The girls have a sadistic fetish for killing men, with multiple previous victims to satisfy their sexual urges for violence, and announce that he will die at dawn.

At dawn, they tie Evan up with a hose, bury him in the hole, leaving only his head aboveground, and tell him they will crush his skull with a large stone. It misses his head, and the girls reveal that the entire ordeal was merely a "game;" they never intended to kill Evan, nor are underage; everything they did was part of a hobby of seducing, victimizing, and ruining the homes of married men with children who cheat on their spouses. Genesis shows Evan the video she recorded with his phone of Bel assaulting him. She uploads it to his Facebook profile. They depart for another victim and take Monkey with them, leaving Evan to his fate. Karen and the kids arrive to find the house ruined.

On April 4, 2014, Keanu Reeves was added to the cast to play Evan Webber. Chilean actress Ignacia Allamand also joined the film.^[9][10] The shooting took place in Santiago de Chile. Eli Roth stated that filming in Chile is easier than in the United States.^[11] Roth wanted the film to have a comedic approach, with physical humor too.^[12]

Knock Knock premiered at the 2015 Sundance Film Festival on January 23, 2015.^[13] Three days later, Lionsgate acquired the distribution rights to the film.^[14] The film was released in the United States on October 9, 2015.^[15]

• List of films featuring home invasions