"Ces soirées-là", a modern pop-rap song that was released in 2000, is performed. Tommy DeVito arrives, introduces himself and explaining how the song is a cover of The Four Seasons' "December, 1963 (Oh, What a Night)". He offers to tell the story of the band, explaining how he started out with the group "The Variety Trio" with his brother Nick DeVito and friend Nick Massi, eventually discovering teenager Frankie Castelluccio and taking him under his wing, teaching him everything he knows ("The Early Years: A Scrapbook"). During these early years Nick Massi helped train Frankie to sing, Tommy went in and out of prison, Frankie changed his last name to Valli, Tommy and Frankie developed a good relationship with mob boss Gyp DeCarlo, and Frankie fell in love with and married Mary DelGado. Musically, the band was still struggling and kept changing their name and sound but without any dramatic success. One day friend and fellow Jersey boy Joe Pesci comes up to Tommy and says that he knows a singer-songwriter who'd make the perfect fourth for their band: Bob Gaudio.
Bob Gaudio takes over the narration, telling the audience that no matter what Tommy says, he wasn't plucked from obscurity by him, since he already had a hit single with "Short Shorts". Bob goes with Joe Pesci to see the band perform, and is immediately impressed by Frankie's voice. Bob performs a song he'd just written: "Cry for Me" on piano, which Frankie, Nick Massi and then Tommy joining in with vocals, bass and guitar respectively. They negotiate an agreement, though Tommy is at first skeptical that Bobby (then still a teenager) will be good for the band. The band eventually gets a contract with producer Bob Crewe but only to sing back-up ("Backup Sessions"). Crewe insists that the band has an "identity crisis" and needs to make a firm decision on a name and a sound. The band name themselves after The Four Seasons bowling alley, and Bobby writes them three songs that finally propel them to stardom: "Sherry", "Big Girls Don't Cry" and "Walk Like a Man". In the wake of their success, Bob also chalks up a personal first by losing his virginity ("December, 1963 (Oh, What a Night)"). The band's success means that they tour a lot more, along the way discovering the girl band The Angels ("My Boyfriend's Back"). Unfortunately, the constant touring strains Frankie's marriage to Mary, and they eventually divorce ("My Eyes Adored You"). The band continues to enjoy chart successes ("Dawn (Go Away)") until after a concert the band is approached by a loan shark out to claim money owed by Tommy ("Walk Like a Man (reprise)").
The second act opens with "Big Man in Town". Nick Massi explains that Bob was so focused on the band's musical success and future that he couldn't see that the band had been in trouble for some time. Tommy's been racking up debts, and a forgotten bill during a previous tour lands the band in jail over the weekend, which strains things between Tommy and Bob. Nick observes that Tommy became jealous of Frankie's success and closeness with Bobby, and attempted to seduce Frankie's new girlfriend Lorraine. The two never confronted each other about it, but the old friendship was not what it used to be. When the loan shark approaches the band for the $150,000 owed by Tommy, Frankie goes to Gyp DeCarlo for help despite Tommy's insistence that he doesn't need it ("Beggin'"). The band, Gyp, and the loan shark come to agreement: Tommy is to be "sequestered" in Las Vegas where the mob can keep an eye on him, and the band will willingly cover the debt, along with an additional half a million in unpaid taxes that Tommy kept hidden from the group. At this time, Nick declares that he's tired of everything and wants out ("Stay/Let's Hang On!").
Frankie takes over narration, explaining that though he owes Tommy a great deal, he's aware that their relationship wasn't ideal, and he never understood why Nick decided to leave. Frankie and Bob find replacements to keep the band a quartet ("Opus 17 (Don't You Worry 'Bout Me)") until Bobby announces that he's never been comfortable in the spotlight and that Frankie should be a single, i.e. Frankie Valli and the Four Seasons. In his personal life, Frankie's relationship with his daughter Francine is strained and he breaks up with girlfriend Lorraine ("Bye, Bye, Baby (Baby, Goodbye)"). Frankie continues to have success thanks to Bobby's songs, and hits jackpot with ("C'mon Marianne") and the almost-never-released ("Can't Take My Eyes Off You") which Bobby fights to get airplay for. Along with the success of "Working My Way Back to You", Frankie and Bobby finally finish paying off Tommy's debts, and Frankie's life is good until his daughter Francine dies from a drug overdose ("Fallen Angel")
Bob Crewe describes The Four Seasons' 1990 induction into the Rock and Roll Hall of Fame, which reunited the original four members on stage one last time ("Rag Doll"). Each member takes a moment to address the audience in turn, explaining his pride at having been with the band and briefly notes what he did afterwards ("Who Loves You").