The musical involves a wedding which brings together the families who are quite, and comically, different.
At the St George Hotel, it is the morning of Rebecca Steinberg and Brian Howard's wedding. Jenny, Rebecca's sister, talks of her nerves before the wedding and how glad she is that she isn't the one having to have all the problems of a bride. Judy and Murray are Jenny and Rebecca's parents ("I Never Wanted This"). Judy, an overbearing, talkative Jewish mother barks commands at Jenny to make sure things are organized. While trying to keep things together, Jenny accidentally calls Rebecca's ex, Marty, and lets it slip the wedding is taking place that day. Marty is convinced that it is a sign that he needs to stop the wedding and talk Rebecca out of it. He rushes to the hotel.
The bride and groom share their mutual excitement and fears ("This Day Opening"). Things go well with a few comedic missteps, including the visible impact of the wedding on the groom's mother, Georgette, who doesn't want to lose her son to another woman. When things seem to settle, the best man, Greg, comes in and announces that Rebecca wants to stop the wedding. Jenny rushes to her side, and Rebecca says that there's a smudge on her wedding dress—it must be a sign.
Annie, the "co-maid of honor", tries to talk to her along with Brian and Greg, until Brian insinuates that Annie has been too controlling over the wedding. That triggers something in Annie, and she runs off with Greg and Brian immediately rushing after her to apologise. Jenny once again smooths over the situation by getting the stain out of the dress. Rebecca and Jenny sing about how much they admire each other, both seeing qualities in the other that they respect ("Perfect"). On the other side of the hotel, Marty arrives and is instantly spotted by Murray, who expresses his joy over seeing him before. Murray admits that he wishes it was Marty marrying Rebecca instead of Brian. He is joined by Judy, the drunken Uncle Morty, and flirtatious Aunt Sheila, who all echo this sentiment ("It Shoulda Been You").
When Jenny sees Marty, she tries to get him to leave. However, Marty appeals to their longtime friendship and all the times that he has been there for her. Jenny reminisces with him, and it's revealed they did get together, but for some reason Marty stopped calling her and she never knew why ("Who"). She reluctantly agrees to give him only a few short minutes with Rebecca. Meanwhile, Brian is having a few moments with George, his father. George says that now that Brian is getting married, he wants to try to have a closer relationship with him—something he and his own father never had ("Back in the Day"). He also gives Brian a pre-nuptial agreement and tells him to consider signing it before the wedding, completely unaware that Marty is hiding behind a plant and has heard their conversation.
The bridal party gets their hair and makeup done, and tensions between the conservative Christian Georgette and open Jewish Judy are running high. Rebecca and Jenny beg Judy to be polite. After Georgette, assuming a cheerful tone, takes a few back-handed jabs at her, Judy emulates her demeanor to hurl shots back at her, including pointing out Georgette's latest face lift ("Nice").
Marty arrives and throws everything into disarray when he announces the plans for the pre nuptial agreement. Rebecca is shocked and runs off. Everyone goes to find her. Jenny starts panicking that the wedding will be over before it's begun. She is reassured by Albert, the wedding planner, that all will be well. He shares stories of all the weddings he's been involved in, pointing out that nothing has ever shocked him or prevented the ceremony from taking place ("Albert's Turn").
Brian tries to confide in Georgette and tells her that he owes everything he has achieved in life to her. As he leaves, Georgette laments losing her son to another woman and wishes he had turned out gay, or at least waited until after she died to find someone ("Where Did I Go Wrong?"). Everyone is still searching for Rebecca; they think she has left, but Albert appears with Rebecca dressed, made up, and ready to walk down the aisle. Judy and Murray admire her, and Judy tells Jenny to go get dressed. As she goes into the bathroom, Judy compliments Rebecca on how stunning she looks before saying she wishes Jenny had found someone. Judy suggests that it would be easier for Jenny to find someone if she lost weight. Rebecca retorts that this isn't fair, as Jenny has had a wonderful life and has done great things. Jenny hears this conversation as she exits the bathroom and is embarrassed. Everyone else leaves, and Jenny finishes dressing. As she does this, she expresses her frustration that even though she views herself as a truly beautiful and valuable person, she struggles to find people that view her as more than 'sort of pretty,' 'kind of sexy,' or just 'nice' ("Beautiful").
Outside the cathedral, Marty shows up one more time to try and convince Rebecca to call off the wedding. Rebecca considers it, but Jenny steps in and tells him to let Rebecca go and be happy. Marty reluctantly agrees, and the wedding goes ahead.
After the wedding, everyone gets ready for the reception. Murray and Judy rejoice that their daughter is finally married. Georgette is in despair and grabs George to head for the bar. Jenny is happy for her sister, but sad for herself. Rebecca and Brian, now in a private room, are just glad things went ahead. ("A Perfect Ending"). Greg and Annie interrupt the newlywed's alone time with champagne in hand to celebrate. They drink, and it is revealed that Greg and Brian and Annie and Rebecca are lovers. Jenny and Albert walk in and catch the two couples in the act. Rebecca tries to explain things to Jenny, but she is interrupted by Judy telling them the reception is about to start.
At the reception, Greg and Annie perform a garish song to their secret lovers declaring their undying devotion under the pretense of having written it for the wedding ("Love you Till the Day"). Rebecca tries to talk to Jenny again, but she is interrupted by Judy, who says it's time for the father-daughter dance. Rebecca tries to resist, but Judy says Jenny can use the dance to search for a husband. That finally sets Jenny off, and she storms away. Judy follows her and tries to see what's the matter while still giving her orders about details of the wedding, but Jenny has none of it. She decides that she's tired of being pushed around and mocked she's going to have a quick fling and throw caution to the wind ("Jenny's Blues"). Marty comes in and she impulsively kisses him. Jenny runs off after she kisses him, but Marty follows her into the bathroom and presses her to explain why she kissed him. She denies it as anything serious, but Marty continues pushing before letting it slip that he knew Rebecca was gay and that's why he couldn't call Jenny back. He didn't trust himself not to tell her after he promised Rebecca he wouldn't. He then says that he never loved Rebecca, but he always loved Jenny. He continues, saying that if she wants him, he wants to start a romantic relationship or whatever she wants. ("Whatever"). The two declare their feelings for each other and kiss. Albert comes in and says that Rebecca is about the tell Judy and Murray that she's gay. They run off to find her. However, what they don't realize is Aunt Sheila was also in the bathroom (hooking up with a waiter) and has heard the whole thing.
Brian and George try to talk Rebecca out of revealing her sexuality on the day of the wedding. Jenny and Marty concur, but Rebecca says she's tired of living a lie. She wants to be who she is and wants to be with who she loves ("A Little Bit Less Than"). Jenny hugs Rebecca and tells her she is strong. Judy and Murray come in to tell them the speeches are about to begin. Rebecca tries to get up the nerve to tell them but ultimately chickens out. As the rest of the family comes in, Aunt Sheila drunkenly announces that Rebecca is gay and, to the delight of Georgette, Brian also confesses his sexuality.
Judy and Marty are shocked and demand an explanation for why they went through with the wedding. Brian and Rebecca explain that they both met in college, and when they found out that each other was gay, they became close friends. As they, along with Annie and Greg, hit 30 they realized they were broke. Brian remembered that he had a trust fund in his name from his grandfather's will. However, the stipulation was that he had to be married to a woman, so he convinced Rebecca to marry him. Rebecca also reveals that she is pregnant with Brian's child as during a period of time when she struggled with her sexuality, she and Brian had a drunken fling. After they got over the shock, Annie and Greg accept it, and they all decide to raise the child together. To the delight of Rebecca and Brian, both of their parents are accepting of their sexuality. Judy and Murray are overjoyed that they will have a grandchild, and Georgette and George take glee in the fact that Brian's grandfather—a racist homophobe—would have hated it.
However, the wedding isn't over. Marty proposes to Jenny, saying he has been in love with her his whole life. Jenny says it is too soon, being that they never even dated. Judy convinces her, saying that she should go for it if she truly loves Marty. She says that love isn't about fairy tales—It takes time, and it involves taking risks and having the little moments that make a relationship come together ("What They Never Tell You").
Jenny and Marty both admit they have flaws, but they love each other, so it doesn't matter ("Perfect/Whatever" Reprise). Jenny says she doesn't have a dress, but Albert has it covered and hurries her off to get ready. Spurred by this quick wedding, Brian proposes to Greg and Rebecca to Annie, and they hurry off to make it a triple ceremony. With both sets of parents now alone together, they reflect over the events of the day, with the two mothers putting aside their differences because they are becoming a family ("That's Family").
Jenny emerges in her dress, and Judy says she looks beautiful. Marty happily agrees and the two marry ("Finale").