Bogota, ColombiaMike Milch (John Gallagher, Jr. ) is on his way to work while driving down a crowded street. He is approached at his car by a man in town who offers him a figure made from an old corncob. Mike buys one from the man. Mike arrives at his job, Belko Industries. The company COO Barry Norris (Tony Goldwyn) pulls up to find armed guards at the entrance asking to see ID. As he enters the building, we meet other Belko employees who have been working there for over a year. Mike goes to his girlfriend, Leandra Florez (Adria Arjona), who is in the process of getting divorced.
Leandra is constantly harassed by Wendell Dukes (John C. McGinley), who doesn't get she's not interested. There is new employee Dany Wilkins (Melonie Diaz), who is told that the employees all have trackers placed in their heads for the higher-ups to keep an eye on them, due to the high number of kidnappings that occur in Colombia. Dany becomes acquainted with Roberto Jerez (David Del Rio) and Leota Hynek (Gail Bean). Not long into the day, the local employees are sent home, and a mysterious voice (Gregg Henry) speaks over the intercom telling the employees that within 8 hours, most of them will be dead. In 30 minutes, they are required to kill two people, or else more will die. The building is then completely sealed off by metal on all the doors and windows. Everybody becomes nervous and tense, while Barry tries to keep everyone calm.
People play it off as a sick prank. Maintenance workers Bud Melks (Michael Rooker) and Lonny Crane (David Dastmalchian) try to melt the metal off with the blowtorch, but the metal is apparently too strong for it to have any sort of effect. Dany, Roberto, Leota, and tech guy Keith McClure (Josh Brener) to go the roof to find cafeteria worker Marty Espenscheid (Sean Gunn) smoking a joint up there with two of his friends. As they discuss the situation, one of Marty's friends dies when the back of her head explodes. The others freak out, while in the lobby, three more people die with their skulls getting blown out. The other employees panic, thinking someone is shooting at them.
Upon further inspection, the employees learn that the trackers in their heads are set to explode. Mike goes into the bathroom to try and cut his out, only for The Voice to speak and order him to stop trying to take it out or they will detonate it on their own. Mike stops at the last second as Leandra begs him to stop. The Voice then states that they are not messing around, and now it is demanded that the employees kill 30 among them within two hours, or 60 will be killed. The employees start to weigh their options, as some of them cannot bring themselves to killing anyone else.
Barry, however, is set to make it home to his wife and kids, so he tries to get security guard Evan Smith (James Earl) to give him the keys to the armory. Evan refuses to do so after he just witnessed one of his fellow guards and friend get killed by the explosive. In the basement, Lonny is freaking out as Bud tries to calm him down. Bud gets too close, and Lonny reacts by hitting Bud in the head with a wrench, leaving a huge dent in Bud's head as he dies. Dany, who was hiding, witnesses the whole thing. Lonny tries to stop her from telling anyone and he grabs her.
Dany kicks herself back and causes Lonny to back his head into a metal bar, killing him. Mike, Leandra, and Evan split up while Barry, Wendell, and Terry Winters (Owain Yeoman) work together to try and break into the armory with a blowtorch. Evan tries to stop them by pulling out his gun, but he relents and must let them continue. Mike takes Evan's gun and shoots the blowtorch tank before he leaves with Leandra and Evan. The three meet up with Keith, Leota, and Roberto to make signs to hang over the building to call for help. The guards in the hangar spot them and start shooting at them, hitting Keith in his hand. The Voice demands them to stop hanging up signs, or their explosives will go off.
Mike and his friends start to head back downstairs, but Mike gets knocked in the head with a fire extinguisher by Barry. In the heat of the moment, Wendell stabs Evan and takes his keys. Evan suffers as bleeds out. Barry and his guys get the guns out and start ordering people to head into the lobby as they get down to less than 20 minutes before their deadline.
They begin to group people according to whether or not they have kids or if they are over the age of 60. Mike is then brought in after he regains consciousness. Barry and Wendell start executing people, and Terry nearly gets to Mike until Dany cuts off the power in the basement. Barry, Wendell, and some of their goons start shooting in the dark while others make it out safely. The Voice speaks over the intercom saying that time is up and they have only killed 29 people. 31 additional people have their heads blown out, including Keith and Leota.
Everyone else hides. The Voice speaks again and says the third round of their game will have the employees with the most kills walk out free. So far, Barry and Wendell have the most kills, so they go off to try and up that count by killing everyone they come across. One woman tries to offer her body to Barry for her life. He responds by twisting her neck backwards.
Barry gets into an elevator as Dany and Roberto try to make their way up on top. Dany gets off the top but Roberto gets crushed by the elevator, leaving Barry stuck. Dany climbs to the other side to get out. Leandra finds Marty and his friend Chet (Abraham Benrubi) in the cafeteria where numerous bodies are laid out. Marty and Chet dug the bombs out of the heads of those who didn't get their skulls blown out.
Wendell shows up and starts shooting. Leandra hides behind a table with Marty and Chet, but those two get shot dead by Wendell. Leandra makes her way over to Wendell and starts swinging an axe down into his face. Mike and Leandra find each other and evade one employee who is flinging Molotov cocktails, and Barry wielding his gun. Barry shoots Dany in the head as she comes out of the elevator. The Molotov guy gets shot too. Unfortunately, so does Leandra. She dies next to Mike before he steps out in a rage and goes after Barry.
The two fight until Mike grabs hold of a tape dispenser and starts bashing Barry's head in. Mike is then the sole survivor. The guards come in and pull him out. Mike is brought into the hangar, where he meets The Voice himself, a man whose face is partially scarred. He tells Mike that the experiment was to observe human behavior in this type of experiment. Mike then reveals that he took the bombs that Marty dug out and slipped them onto the guards and The Voice. He runs to the control panel and sets them off, then shoots two other goons. The Voice tries to talk Mike out of it, but he has none of it and shoots The Voice man dead.
Mike walks out of the building. The camera pans out to reveal other screens from dozens of other Belko companies where other survivors have completed their own ordeal. Another voice then commences "Phase 2. ".