Next Thing You Know begins as a young bartender, Waverly, unlocks the front gate of her dive, flicks on the few working lights, and plugs in the neon sign signaling to revelers that she's open for business (PRELUDE). Like moths to a flame, twenty-somethings from across Manhattan arrive to live it up at this LITTLE BAR ON SULLIVAN STREET. Waverly seems to have it all—a charming boyfriend, Darren, a wonderfully devoted best friend, Lisa, and the freedom to go after all of her dreams. But when her day job as a law clerk offers her a full-time position, she sees it as a harbinger of doom—if she takes the job she won't be able to pursue her acting career anymore. Lisa suggests that perhaps it's time for things to move on, and Lisa herself is thinking about moving out to LA. Suddenly, everything's changing when Waverly just wants it all to STAY.
As life threatens to grow up around Waverly, a young charmer named Luke enters her bar. He sees Waverly is upset, and offers to take her home to his place for a wonderful evening followed by his MORNING AFTER OMELET. Waverly is flattered, but turns Luke down, and heads home to her boyfriend, Darren.
With the job offer on the table, her best friend on the verge of leaving, and Luke's advances on her mind, Waverly arrives home. She pours her heart out to Darren, and he mistakes her confusion for something it's not, and tries to calm her down—by proposing. It's all too much for Waverly, and she tells Darren that not only does she not want to get married, she wants to take a break from everything. Darren makes a desperate plea to get her to stay (AS GOOD AS I GET). Waverly still walks out the door, leaving Darren crushed.
Weeks later, Luke and Darren are sharing an office where Luke is a salesman and Darren is a temp. One serendipitous afternoon, they begin small talk. Luke quickly learns of Darren's broken heart and takes him to a local watering hole (WE'RE GONNA GO OUT) where he teaches him THE WAY TO GET A GIRL. While Luke and Darren hunt through the bar, Waverly and Lisa share drinks and conversation in a darkened corner. After months of deliberation Lisa decides it's finally time to leave New York, because today was the first day she took the subway over the MANHATTAN BRIDGE and didn't notice the view.
Waverly heads to the bar to get more drinks for her and Lisa, when she runs into Darren, who quickly ruins any chance he had at winning Waverly back by utilizing the techniques Luke has just taught him (HOW 'BOUT YOU?). Although hurt by Darren's behavior, Waverly finally feels free to grab her youth with both hands, and takes the first guy she can find home with her—Luke (DON'T SAY ANOTHER WORD). Darren is left to stumble home by himself (WE'RE GONNA GO OUT REPRISE). The next morning, they all awake HUNGOVER from one evening of indulgence in a decade of decadence.
Weeks later, Luke finds himself falling hard for Waverly, but she finds every reason to keep things casual—including not committing until Luke stops smoking. So, in an attempt to mature for Waverly, Luke tries desperately to quit (AND I BREATHE). Waverly is unappreciative of Luke's attempts to grow up—the whole reason she liked him was because of his eternal youth—and in the ensuing fight she accidentally calls him Darren, revealing to herself that maybe she's not over Darren after all (IF SHE WERE COMING HOME/I WISH THERE WERE A REASON).
Waverly laments to Lisa that she's torn between her new job and her life as an actress, Darren and Luke, growing up and staying young forever—and Lisa's had enough. She tells Waverly that choosing is part of growing up and that YOU CAN'T BE EVERYTHING YOU WANT. Waverly finally decides to choose a path forward, even if it means closing doors along the way (NEXT THING YOU KNOW). Meanwhile, Luke and Darren realize that they are both in love with the same woman. Luke tries aggressively to win her back, but Darren just tells Waverly how much he loves her, and lets her make her own choice (ALL THAT I WANT IS YOU). Luke returns to his youthful ways for a little while, Lisa moves on to LA and Waverly and Darren enter the next chapter of life together. It's not how they planned it, but it's even better than they dreamed (NEXT THING YOU KNOW FINALE).