Billy, played by Michael Douglas, along with his childhood friends, Paddy, portrayed by Robert De Niro, Archie, brought to life by Morgan Freeman, and Sam, played by Kevin Kline, find themselves navigating the complexities of their senior years after a lifetime of friendship rooted in Flatbush, Brooklyn. While Sam and his wife Miriam lead a predictable existence in a Florida retirement community, Archie, a retired Air Force member dealing with the aftermath of a stroke, lives with his overly cautious son Ezra in New Jersey. Paddy, steeped in mourning from losing his wife, Sophie, more than a year prior, resides alone in a Brooklyn apartment, while Billy enjoys success in Malibu, California, sharing his life with a much younger girlfriend, Lisa.
As Billy prepares to tie the knot with Lisa, he stumbles upon an old bottle of scotch that he and his friends pilfered during their youth, reigniting memories and camaraderie. This inspires the group to throw Billy a bachelor party in Las Vegas, an idea quickly supported by Sam despite his marital commitments to Miriam. With an opportune secret rendezvous, they manage to pick up Archie and Paddy, setting the stage for an unforgettable reunion in Sin City.
Upon arriving in Vegas, tensions flare between Billy and Paddy due to past grievances, prompting Archie to step in and urge the group to celebrate Billy's upcoming wedding together. Their plans take a turn when they discover their original hotel is undergoing renovations. With a little help from Billy’s assistant, they secure a room at the Aria Resort and Casino where the wedding is set to take place. While hoping for a dramatic weekend, the friends encounter a lounge singer, Diana, who captivates them—all while Paddy grapples with lingering resentment towards Billy for missing Sophie’s funeral.
In a spirited attempt to enjoy their time, they judge a swimsuit competition, and soon after, Archie finds himself on a surprising winning streak at blackjack, leading the friends into chaotic yet thrilling misadventures. Amidst the festivities, Billy and Diana’s connection deepens as he learns about her dreams and struggles. As the night escalates, personal dilemmas intertwine with humor and heartfelt moments, shedding light on the significance of their friendship and the challenges of moving forward.
As the party spirals, tensions rise again. Billy's past choices about Sophie haunt the reunion, leading Paddy to confront him about feelings that resurface, but amid the chaos, he begins to consider a future beyond his grief. The bachelor party unfolds with unexpected twists, leading to humorous encounters, deep discussions, and the occasional confrontation that showcases the bonds of friendship and the trials of life.
As realizations hit hard, Paddy forces Billy to confront the implications of marrying Lisa, ultimately leading to a heartfelt reconnection with the past. Battling their fears of aging and loneliness, the friends rally around Billy, urging him to pursue what he truly desires. In a climactic twist, Billy seeks out Diana to declare his feelings, bringing a close to an emotionally charged weekend.
Months later, as life settles into a new rhythm, Billy and Diana reach out to announce their plans to wed, while Sam is preoccupied in his own world, demonstrating the unpredictable journey of love and friendship that binds them together. The film concludes with the friends cherishing their memories and embracing the future, a testament to enduring bonds that age gracefully like fine scotch.
