The movie opens at the New Jersey State Police Academy in the autumn of 2008. Paul Blart (Kevin James), a single father, tries out to be a cop. He does surprisingly well on the obstacle course despite his size, but passes out due to hypoglycemia, and fails the course.He lives with his mother and daughter, who tries to find him a girlfriend on a dating website, as his wife had previously left him, and he had been feeling lonely ever since. He tries to make ends meet as a security officer at West Orange Pavilion Mall. He takes the job very seriously, but receives no respect from the patrons. The next day, Blart is introduced to a new trainee named Veck Sims. Blart takes him on a tour of the mall, teaching him the basics. While patrolling the mall, Blart sees – and falls for – a kiosk owner, Amy. After a few attempts to muster up courage, Blart tries to talk to Amy, but awkwardly asks for hair products instead. Upon their second “meeting”, Blart offers to give Amy a ride to her car on his Segway, and they agree to meet later in a bar where the mall staff are regular patrons. Unfortunately, Blart accidentally gets drunk, and humiliates himself in front of everybody present. Thinking he has ruined his chances with Amy, Blart returns home in low spirits. Later, Blart’s co-worker offers him his daughter’s phone so Blart could call Amy.
On Black Friday, a group of robbers disguised as Santa’s helpers “seize the mall” while Blart is inside the arcade playing Rock Band. While in the arcade, a foreign teen named Pahud calls the cell phone looking for the co-worker’s daughter. Pahud believes Blart is her lover but he explains that he is only borrowing the phone and gives the boy positive advice (basically to just relax about this girl). Pahud begins to admire Blart and they become quick friends. The robbers chase most of the shoppers and staff outside. However, some of the mall staff, including Amy, are taken hostage inside the mall’s bank. Veck reveals himself to be the ringleader, and intends to steal thirty million dollars from the mall by obtaining the codes to the mall’s credit card machines.
The West Orange Police, led by Sgt. Howard, surround the mall, but are unsure of the actual conditions inside. Meanwhile, Blart realizes that the mall has been taken over, and he makes contact with Sgt. Howard. However, the moment he finds out Amy is among the hostages, Blart resolves to take the matter in his own hands. He begins incapacitating the robbers one-by-one, and recording the stolen credit card codes. Outside, SWAT Commander Kent arrives to take control over the operation, and he ultimately dismisses Blart’s actions; Also he has known Blart as Kent has been cruel to him since in high school. He shows that he is much more willing than Sgt. Howard to raid the mall, regardless of the hostages’ safety. While Blart is busy fighting the robbers, his daughter, Maya, arrives at the mall to deliver his dinner, but she is quickly taken hostage by Veck. She then meets Amy, and tells her how Blart really feels about her.
Finally, Blart manages to incapacitate all of Veck’s henchmen, but Veck discovers Maya’s identity, and he tries to force Blart to give him the codes in exchange for her life. Instead, Blart distracts Veck with a mannequin, and tries to free the hostages, but fails. He is forced to give Veck the codes, and Veck escapes the mall with Amy and Maya in tow. Blart follows them, with Commander Kent finally on his side. Blart finds out that Veck is headed to a nearby airport, and ambushes him there, successfully restraining him. However, Commander Kent then attempts to kill Blart and points a gun at his head, and reveals that he was working with Veck the entire time. Fortunately, Sgt. Howard had followed them, and quickly arrests Commander Kent after Blart’s boss, Chief Brooks shoots the gun out of Kent’s hand. Blart and Amy are re-united and Blart admits his feelings for her and she kisses him. Howard offers to let Blart become a full-fledged police officer. Blart steadfastly refuses, saying that his place is helping people in his mall. During the end credits, bonus scenes show Blart and Amy getting married in the mall with all of their co-workers and friends there, including Pahud.