Paul Blart (Kevin James) is a mild-mannered, occasionally-square, overweight security guard who works at patrol in a shopping mall called the West Orange Pavilion Mall located in the township of West Orange, New Jersey. He lives with his elderly mother Margaret (Shirley Knight) and estranged daughter Maya (Raini Rodriguez). He dreams of being able to join the New Jersey State Police, but always fails the entrance test due to his hypoglycemia. While on patrol at work riding his Segway PT, Blart notices a new wig kiosk and the kiosk's vendor, Amy Anderson (Jayma Mays). Back at base, Blart has to take the new guy, Veck Simms (Keir O'Donnell), out on a patrol to train him. At the end of Blart's shift, he becomes acquainted with Amy. He escorts her to her car and she invites him to join her, Veck and some friends at American Joe's, a bar located in the mall. Blart accepts and attends the event. However, at the event he gets extremely drunk after drinking too much margarita (which Blart had mistaken for lemonade) during his nacho eating contest with Leon (Jamal Mixon) and crashes the stage that the band is playing on, culminating with him falling through one of the windows.
A few days later, on Thanksgiving, Blart feels as though he embarrassed Amy and that she doesn't like him anymore and Maya tries to cheer him up. On Black Friday at the shopping mall, Amy, Stuart (Stephen Rannazzisi), and other colleagues are in the bank to cash their paychecks. While a gang of criminals (in disguise of Santa's Little Helpers) suddenly break through the service entrance and take over the mall, Blart is distracted by playing Rock Band in the arcade zone. The criminals clear the mall and take Amy, Maya, and other colleagues hostage in the bank. Veck is revealed to be a double agent and the ringleader of the hijackers, and plans to use the credit card codes from all the stores in the mall to steal $30 million, fly to the Cayman Islands and take the hostages with him.
As Blart leaves the arcade, he realizes the mall has been taken over by a gangs of criminals and calls the police, who show up outside the front entrance of the mall. Police sergeant Howard (Adam Ferrara) instructs Blart to leave the mall immediately, but when Blart sees Amy's car in the parking lot, he realizes he must return to save his friend and daughter. SWAT team soon arrives and Commander James Kent (Bobby Cannavale) who was a high-school bully of Blart, takes control of the situation. Blart is vastly outnumbered and physically outclassed but fights against the gang members (who use various forms of martial arts and parkour techniques) through improvised measures, Maya shows up to bring Paul his dinner, but she is intercepted by Veck's henchmen and taken away to the rest of the hostages. After Blart works his way through all of Veck's underlings, he tries to extract the hostages, but fails and is captured. Veck demands to have the credit card codes and Blart is forced to fling him the phone that contains the codes. Veck escapes with the codes and holds Amy and Maya at gunpoint.
Blart steals the mall's display minivan and tracks Veck and Kent to the airport, where Veck, Kent, Amy, and Maya attempts to escape in a Cessna Citation III. After a brief fight, Blart cuffs Veck, but Kent pulls a gun on him, and reveals that Kent was working with Veck the whole time. However, before Kent can kill Blart, police arrive in time to shoot and arrest Kent. The next day, Maya is proud of Blart and he gives Amy a birthday card; Amy kisses him. Sergeant Howard offers Blart a job with the West Orange Police, but he decides that his place is protecting the people at the mall with Amy and the rest of his friends. During the closing credits, bonus scenes shows Blart and Amy getting married in the mall with Blart's family and friends there, riding Segways.