Billy (Michael Douglas), Paddy (Robert De Niro), Archie (Morgan Freeman) and Sam (Kevin Kline), now in their late sixties, have been best friends since childhood. When Billy, the last single member of their group, finally proposes to his much-younger girlfriend, the four decide to throw the ultimate Las Vegas bachelor party.
However, upon arriving, the four quickly realize that the decades have transformed Sin City. In their day, the Rat Pack played the Sandsnow Cirque du Soleil rules the Strip. They soon find themselves on an unexpected adventure as Sam attracts the attention of a drag queen and Archie wins big at blackjack, which gets them an upgrade to the hotel's most lavish suite.