The film begins with a person committing suicide by jumping from a building narrated by Ramirez (Jacob Vargas) who mentions that his mother tells him stories of the Devil roaming the earth, and it always begins with a suicide. Detective Bowden (Chris Messina) is called to the scene to aid in the investigation. Bowden is a recovering alcoholic devastated by the death of his wife and child in a hit-and-run accident by a driver who was never caught. As this is happening, five strangers, who have committed various crimes in the past, step onto an elevator located within the same building where the suicide has taken place.
The five strangers include Ben Larson (Bokeem Woodbine), a temp security guard with a violent past; an elderly woman (Jenny O'Hara) who is a compulsive thief; Vince McCormick (Geoffrey Arend), a mattress salesman who moonlights as a con artist; Tony (Logan Marshall-Green), a former mechanic who served in the U.S. military during the War in Afghanistan who is now seeking employment within the building; and Sarah Caraway (Bojana Novakovic), a greed encompassed heiress meeting with her lawyer in the building.
Strange things start to occur beginning with the elevator becoming stuck between floors. Then, after the lights go out, Sarah is inexplicably wounded on her back. The remaining occupants of the elevator quickly begin to suspect Vince of having committed the assault. Slowly, one by one, the five strangers start to die. Ramirez and his colleague also see a strange devil-like face appear on the screen of the security camera. First, Vince is killed by a shard of glass from a mirror which slices his jugular vein. Detective Bowden, sensing a connection between this and the man who earlier committed suicide in the same building, is compelled to further investigate. Checking the building's guest log, Bowden finds that only four people have missed their scheduled appointments that day: Sarah, Vince, Ben, and Janecowski. The investigators misinterpret the latter as Jane Cowski and assume this is the old woman's name, leaving Bowden suspicious of Tony who appears to be the only undocumented occupant.
With the help of the building's security team, Bowden examines security footage and discovers that the old woman had stolen a wallet prior to entering the elevator. The office building's repairman is sent down the elevator shaft to fix it, but plummets to his death. During a power outage, the old woman is found hanging by the neck from an electrical cord dead, during the onslaught of the power and the gripping darkness a flash of an evil figure is seen. Sarah and Ben turn on Tony, while Bowden begins to suspect that Sarah's husband has hired Ben to kill her. A security guard inspects the basement and electrocutes himself attempting to secure a hot fallen wire. The lights go out again and Ben is dead with his neck completely twisted around. Each thinking the other must be responsible for the murders, Tony and Sarah prepare to fight each other with broken glass, but Bowden seemingly manages to calm them. As Sarah prepares to take out a shard of glass hidden in her back pocket, the lights once again go out and her throat is slashed. The mystery seems solved, when a tattooed woman arrives and informs Bowden that Tony is her fiance, and was at the building for a job interview. Only then is his full name revealed to be Tony Janecowski.
The old woman suddenly rises as she is then suddenly taken by the devil and appears behind Tony. It is now apparent that she is the Devil who has taken a human form. Having dispensed of the others, The Devil tells Tony it's his turn to die and Tony accepts it, even offering to be taken instead of Sarah, who's still struggling to breathe. Detective Bowden watches through the CCTV as Tony confesses to killing two people in a drunken hit-and-run accident and showing that even though all the other people in the elevator have sinned it is he who truly represents the devil among them as his sin is so bad. Tony had been trying to grab another beer while driving and had not seen where was going. Tony says "I'm so sorry." and Bowden had a car wash coupon that says "I'm so sorry" on the back. At which point the Devil is forced to spare him (as he confessed his sin and apologized for it) and disappears as the firemen finish breaking into the elevator. Detective Bowden realizes Tony is the one who killed his wife and son, but expresses his forgiveness en route to the police station following his arrest.
The film ends with Ramirez telling the audience that his mother always reassured him at the end of her stories, "If the Devil is real, God must also be real."