At a bustling party, college freshman Charlie Greengould bemoans his insecurities (“Join the Party”). Intimidated, he leaves the party and bumps into his friend Lila. As Lila divulges the details of a just-happened hookup, Charlie reveals to the audience his feelings for Lila and his frustration at his inability to act on them (“What Went Down / Be Like That”). Charlie and Lila say goodbye, and Charlie goes back to his dorm room to sleep.
The next morning, Charlie wakes to find a person who looks exactly like him sitting in his room: Parallel Charlie (P.C.), a suaver and more confident version of Charlie from an alternate universe. P.C. makes a proposal: he’ll give Charlie the life he wants by pretending to be him (“The Proposal”). Panicked, and needing to process what he has just heard, Charlie runs outside, where he bumps into two bro-y acquaintances. They congratulate him on having very publicly made out with an attractive girl at the previous night’s party—something, of course, that Charlie did not do. As the bros describe what they saw, Charlie realizes that it must have been P.C. with the girl, and that he, the real Charlie, has already had his reputation boosted because of it (“The Fantasy Made Real”). Seeing what’s possible with P.C.’s help, Charlie takes credit for the hook-up, runs back to his room, and agrees to P.C.’s plan.
Weeks later, Lila is in Charlie’s room, making out with P.C. (who she of course believes is Charlie), as the real Charlie hides under the bed. Lila tells “Charlie” (actually P.C.) how excited she is about her newfound attraction to him (“I’m Amazed at the Way You’ve Changed”). But when Lila leaves the next morning, Charlie confronts P.C.: he’s ready to take his life back into his own hands. When P.C. can’t convince Charlie otherwise, he explodes with anger, fully revealing his nefarious nature (“You’re Nothing Without Me”). He storms out of the room, threatening to do something that will really hurt Charlie.
Alone in his room, a terrified Charlie waffles. He knows he should go out and try to stop P.C., but he’s too scared. Charlie looks back on moments from his past when he was frozen with nervousness and later regretted it (“Work Up the Nerve”). But right as he’s about to open the door and leave his room, P.C. bursts back in and gloatingly reveals that he has just taken Lila’s virginity. Furious, Charlie attacks P.C., and they fight physically. Although Charlie is far outmatched, he manages to give P.C. a black eye, and P.C. runs out of the room. Charlie follows him quickly enough to see him disappear into a strange abyss——an abyss Charlie realizes must be the portal into the parallel universe. Once again, Charlie tries to work up the nerve to stop P.C. (“Work Up the Nerve [Reprise]”)—but this time he does so successfully, and he jumps into the portal as Act One ends.
Charlie finds himself on campus in a parallel universe, with P.C. nowhere to be found. Confused, Charlie does the only thing he can think of, and wanders to “his” dorm room. But when he knocks on the door, a girl answers—and he quickly realizes that she is yet another version of him (“We Must Be Each Other”). Charlie and G.C. intuit that there must be many parallel universes and many Charlies, given that they already know about three. Feeling an immediate bond—they are, after all, versions of the same person—G.C. agrees to help Charlie look for P.C. He shows her the portal, and they jump through together.
Charlie and G.C. enter the next universe only to find (that universe’s) Lila grieving over the loss of (that universe’s) Charlie, who is missing and presumed dead (“Lila’s Lament”). Suspecting that P.C. had a hand in this Charlie’s disappearance, our Charlie and G.C. realize that they must find and stop him (“One Less Charlie”). Since they cannot let anyone in their current universe see Charlie—as in this universe, he’s dead—they run off campus to plan their next step.
In a moment of comic relief, Charlie and G.C. awkwardly succumb to the growing sexual tension between them and begin to make out. Freaked out, Charlie pulls away and debates whether or not to give in to his bizarre desires (“The Opposite of Opposites Attract”), eventually deciding to go for it. The lights go down as Charlie and G.C. lunge for each other.
When the lights come up, our two protagonists brush themselves off and continue on their quest to find P.C., traveling through many universes and briefly encountering several other versions of Charlie (“Multiverse Montage”). Charlie and G.C. finally find P.C. (recognizable by his black eye), and they jump him. Pinned down, P.C. begins to talk: he found the portal at a young age, he explains, and got trapped within it (“Lost”). Forced to watch countless other versions of him grow up and lead fulfilling lives, while unable ever to return to his own life, P.C. became warped by torment and jealousy. Charlie finds himself unexpectedly sympathetic towards his double, as he understands what it’s like to wish your life was different, but G.C. has no such sympathies.
Then Parallel Charlie plays his trump card: years of traveling through alternate worlds have taught him how the navigate the portal, and he’s the only one who knows how to get Charlie and Girl Charlie home. Facing no other choice, they strike a bargain with P.C., leaving him alone in exchange for their safe return. Although he understands now that he has to stop running from his own life, Charlie still has strong feelings for G.C. Before they leave, he tells her how much he wishes that they didn’t have to part forever (“In a Perfect World”).
Back in his own universe, Charlie rushes to meet Lila. He starts to tell her the truth about his experiences—but of course he seems crazy, and she doesn’t believe a word he says. Realizing that he will never be able to tell anyone about what he has experienced, Charlie pretends that his tale was just an elaborate way to explain the personal developments he has been through (“It Was Just a Metaphor”). The curtain goes down, and life goes on.