Four days after the events of Rush Hour, LAPD detective James Carter is on vacation in Hong Kong, visiting his friend, HKPF Chief Inspector Lee, as he was sent along with Lee in saving the life of the Chinese counsel's daughter in Los Angeles. The fun is put on hold when a bomb explodes at the United States Consulate General, killing two undercover U.S. Customs agents inside. Inspector Lee is assigned to the case, which becomes personal when it is discovered that it somehow involves Ricky Tan, his late police officer father's former partner. Tan, who was suspected of having a role in Lee's father's death (though the connection was never proven), is now a leader of the Triads. This, however, causes a rift between Lee and Carter, the latter who simply wants to enjoy his vacation and not get into any danger. The two encounter Tan at a massage parlour.
The U.S. Secret Service, led by Agent Sterling, and the HKPF soon get into a fight over the jurisdiction of the case. Suddenly, the nearby room that Carter was in is bombed, causing Lee to believe he's dead and grieve for him. Carter is revealed to be alive, leaving the room before it exploded. He and a relieved Lee cross paths at Tan's yacht where he is holding a dinner party. Tan scolds his underling, Hu Li, who then leaves as Lee and Carter confront Ricky Tan. Just as Ricky Tan asks for protection, Hu Li shoots him and makes her escape in the chaos. An angry Sterling holds Lee responsible for Tan's death, and orders him off the case. Carter is ordered back to Los Angeles for involving himself and Lee volunteers to take him to the airport. However, at the airport, Carter gets Lee to return to LA with him.
On the plane, Carter tells Lee that in every large criminal operation, there is a rich white man behind it and that man is Steven Reign, a billionaire Los Angeles hotelier whom Carter saw acting suspiciously on Tan's boat. They set up camp outside the Reign Towers, spotting a U.S. Secret Service agent named Isabella Molina, whom Carter met earlier in Hong Kong. After a few misunderstandings, Molina tells the two men that she is undercover, looking into Reign's money laundering of $100 million in superdollars.
Lee and Carter pay a visit to Kenny, an ex-con known to Carter who runs a gambling den in the back of his Chinese restaurant. He tells them that a usually broke customer recently came into his establishment with a suspicious amount of hundred-dollar bills. Carter confirms that they are Reign's counterfeits and they trace the money back to a bank. The mobsters are waiting for them and knock the two cops unconscious, with Molina looking on. After arriving in Las Vegas, Lee and Carter wake up inside one of the mob's trucks and escape. After finding out where they are, they realize that Reign is laundering the $100 million through the new Red Dragon Casino.
At the Red Dragon, Lee and Carter split up. Lee attempts to find the engraving plates which were used to make the counterfeit money, while Carter makes a distraction to help Lee sneak past the security. However, Hu Li captures Lee and takes him to a room where it is revealed that Ricky Tan faked his death. When Tan departs, Molina tries to arrest Hu Li but Hu Li easily over-powers her and Molina is shot. Carter continues to fight Hu Li in a comical manner and knocks her out, while Lee heads to the penthouse to prevent Tan from escaping with the plates. In the penthouse, Reign opens the safe and takes the plates, running into Tan as he leaves. Reign tries to back out of the deal but Tan stabs him to death. Lee and Carter arrive and a scuffle between them and Tan ensues after Tan admits that he killed Lee's father and mocks him for only asking Tan to spare Lee's life before he died.
Tan falls to his death when Lee kicks him out of the window. Hu Li enters with a time bomb forcing Lee and Carter to grab onto the decoration wires. The two escape on the makeshift zipline as Hu Li kills herself in the explosion. Later, at the airport, Molina thanks Lee for his work on the case, and she kisses him for a short time, while a jealous Carter watches from afar. Having originally planned to go their separate ways, Lee and Carter change their mind when Carter reveals he won a large amount of money at the casino and the pair decide to head to New York City to indulge themselves.

Question: Why did Lee and Carter go to Las Vegas?
Answer: They found out Steven Reign was going to launder the counterfeit bills through his casino The Red Dragon there.