Paul Blart stops the criminals and kisses Amy. He is told that he can join the real police anytime he wants.
(Beware, many plot holes!) Veck Sims (the Mall Security recruit) turns out to be the leader of the heist and takes several people, including Amy, the girl Paul has his eye on, hostage. Paul is playing video games in a closed arcade when the heist starts, so everyone is gone except the criminals, hostages and him, leaving him as the only person inside who can stop them.
Paul is talked out of doing anything by the police Sergeant and he rides his Segway out of the mall, but sees Amy's car still in the parking lot, and figures that she must be a hostage, so he rides back into the mall.
Veck announces his plan to all the hostages for no good reason - apparently all the stores in the mall all have different security codes on the credit card machines, and he needs them all to take all the money from the bank. The criminals write the codes on their arms using "invisible" ink that you need to shine a UV light on to see - but that doesn't matter because all the criminals carry UV lights anyway.
During this time, his daughter, Maya, shows up to bring him his lunch even though it is well into the night. She uses a security pass she got from seemingly nowhere to just walk in a door that apparently none of the criminals or police can see or protect from entry. She is captured by Veck and taken as a hostage.
Commander Kent, the leader of the SWAT team, arrives and takes command of the situation from the police.
Paul Blart takes out the criminals one by one, saving all the codes onto his phone for no real reason while the SWAT team does nothing and just watches as he does everything.
Eventually, it is down to just Paul and Veck. Paul hands over the phone and Veck runs away with
Amy and Maya. He throws them into a van and drives away. Paul chases them with the SUV that is parked on display in the middle of the mall, because the keys are apparently in it. He drives through a window and is shot at, but he isn't hurt. Commander Kent gets into the passenger seat and they start to chase down Veck.
Eventually they chase him down to the airport and Veck pushes Maya and Amy into a randomly awaiting plane. Paul gets into a fist fight with Veck and tricks him by pretending to pass out, then kicking his feet out from under him. Paul arrests Veck, only to be held up by Commander Kent, who out of nowhere is now involved in the heist.
Commander Kent demands the phone, but Paul smashes it. He laughs and says that's okay because he has all the other criminals in custody and so has all the codes, so apparently the rest of the SWAT team is in on it too or else he wouldn't be able to get the codes. Kent aims a gun at Paul, but Kent's arm is shot by the Mall Security leader who, up until right this moment, never carried a gun. The rest of the police arrive immediately after and arrest all the criminals.
Paul finds Amy and hands over a birthday card he bought from the mall's Hallmark store during the heist. She kisses him in return, and they hold hands. The police Sergeant comes over and tells Paul that he can join the real police whenever he wants to.