The film starts with Cedric (Kevin Hart) saying how Las Vegas has always been a place for casinos and showgirls, and now it's the perfect place for couples to get married.
He and his friends are all gathered there for the wedding of Michael (Terrence J) and Candace (Regina Hall), with her son Duke (Caleel Harris) joining them. The other couples arrive with more than their fair share of baggage. Zeke (Romany Malco) and Mya (Meagan Good) arrive, with Zeke's womanizing past constantly being brought up by old friends and former flames, including the concierge at the hotel that recognizes him and throws her drink in his face. Bennett (Gary Owen) and his wife Tish (Wendi McClendon-Covey) are trying to get a spark back in their marriage. Jeremy (Jerry Ferrara) and Kristen (Gabrielle Union) have since been married and are now trying for a kid. Dominic (Michael Ealy) and Lauren (Taraji P. Henson) are reunited after spending time away from each other, and they make out in the limo. Finally, Cedric pulls up to the hotel in a gaudy sports car, happily getting away from his wife Gail (Wendy Williams).
The men and women separate to enjoy their day. Cedric, being the best man for the wedding, books himself a lavish suite and intends to go all out for a wild weekend in Vegas. Meanwhile, Michael's overbearing mother Loretta (Jenifer Lewis) doesn't approve of his engagement with Candace, still thinking no woman is good enough for her son. This, along with Candace's desire to have the perfect wedding, heaps on the stress. Loretta even takes over the bachelorette party duties, even though Lauren is the maid of honor. The women start to have a party for Candace, complete with their own blow-up doll (named Idris), until Loretta makes up her own party. It's boring.
Among other problems plaguing the couples include Lauren being called by her boss Lee (Kelsey Grammer), who tells her she is being considered for a COO position in New York, meaning she will be separated from Dominic. Kristen tries to get Jeremy to have sex with her and try for a kid by roleplaying "Game of Thrones" characters, but Jeremy has too many reservations over being a dad. He's also not thrilled about having to give up smoking weed, and he instead brought weed breath strips. Mya is ready to marry Zeke, but he clearly shows hesitation over the matter.
While the ladies have to deal with Loretta, Lauren and Candace conspire to set her up with Candace's Uncle Eddie to get Loretta off their backs so they back enjoy the night. Loretta takes all the ladies out to dinner that night and then plans to take Candace to see Dionne Warwick. That's when Uncle Eddie (Dennis Haysbert) shows up at the right moment and begins flirting with Loretta. The ladies pass him onto her to go see Dionne Warwick, so they can leave to have fun. Elsewhere, the guys are hitting the casinos, with Cedric trying to score as much as he can. They run into Michael's old frat brothers Isaac (Adam Brody) and Terrell (David Walton), who just want to party. Dominic is then pulled aside by a chef who offers him a position there in Vegas, which could possibly jumpstart his career as a chef.
The ladies give Tish a makeover (complete with a montage) to undo her frumpy housewife look to a more alluring style. They then take the guys' party bus to the club, forcing the guys to either take a Chippendale-type bus, or walk. They choose the latter. On the bus, the ladies find Jeremy's weed strips and take them, while Lauren gets a call from her boss who officially makes her COO.
Cedric finds himself in hot water when Gail finds out that he's been using her card to pay for his expenses, running her up to $40,000 in charges. He tries to win it back at the casino, thinking his lucky number is 15 (because that's the age when he lost his virginity and the number of times he and Gail split up). He puts all his chips and money on the table, and with his back turned, Zeke pulls the chips away to leave only one to spare Cedric the loss. However, the ball does land on 15, and Cedric is pissed at Zeke for ruining his chance at a win. Meanwhile, the ladies start to feel the effects of the strips, and they start to make an impromptu video to Bell Biv Devoe's "Poison".
Turning to a last resort, Cedric gets all the guys together to the club (the same one the ladies are at) for amateur's night, dressing in their underwear to try and score some money. This turns into a disaster when Michael spots Candace getting a lap dance. He charges to attack the dancer, leading to an all-out between the guys, ladies, and the strippers.
The whole gang is thrown in jail for the night, which happens to be hours before the wedding. They all try to call someone, with no luck. Michael tries for his mother, but she's spending the night with Eddie in her room. Cedric tries for Gail, but she's already shacking up with someone else: Drake (appearing as himself). Bennett calls his own mother but has his kid pick up. Jeremy sees the way he talks to his kid and is influenced to become a dad. The rest of the guys blame Cedric for getting them into this mess from the moment he planned the weekend.
Loretta sees the messages from Michael on her phone and rushes to bail him and his friends out.
Everybody rushes to the venue for the wedding, but since they were too late, their spot has been taken. Candace runs away crying, and Michael leaves without a word. During this time, the couples resolve their problems. Bennett, aroused by Tish's new look, takes her to bed. Jeremy decides he is committed to being a father, and Kristen reveals she might be pregnant (she claims to be a week late). Dominic and Lauren admit that they were considered for good jobs, but they each turned them down to stay close to each other. Zeke apologizes to Mya for everything that's been going on, and he finally proposes to her. She says yes.
Loretta goes into Michael and Candace's room to continue trying to send Candace away. Michael walks in on this and orders his mother to apologize to Candace. He declares that he will marry her whether she likes it or not, and that she is not invited to the wedding. Candace talks to Michael and says that he needs to apologize to his mother, because as a mother herself, she would be heartbroken if her own son did not let her go to his wedding. Recognizing her mistakes, Loretta makes the first move and apologizes to Candace. The three of them hug.
Cedric packs his bags, feeling like a failure as a best man. However, with the help of his personal butler Declan (Jim Piddock), he is able to find another venue. Candace and Michael are married, and everybody is happy.
As the gang leaves the hotel, Cedric, having had enough of Vegas, drops his last dollar on the floor. Bennett picks it up for him, but Cedric tells him to go away. Bennett then puts the dollar in a slot machine. He scores $100,000. Cedric runs in to try and claim this since it was his dollar, and everybody else tries to pry him away from fighting Bennett.
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