The film begins with audition tapes featuring the talented main cast: Dan Aykroyd, Chevy Chase, John Belushi, Gilda Radner, Jane Curtin, Garrett Morris, and Laraine Newman, leading up to their significant debut.
On the evening of October 11, 1975, at 10:00 PM in New York City, creator Lorne Michaels arrives at 30 Rockefeller Plaza, gearing up for the inaugural live broadcast of "Saturday Night," a new sketch show he has devised. With a compressed timeline of 90 minutes to air, he faces numerous challenges concerning the cast and crew. Assisting him is Dick Ebersol, NBC’s Director of Weekend Late Night Programming, who has been a strong supporter, along with Neil Levy, Lorne's cousin and assistant. Comedian Andy Kaufman also arrives on set, while potential cast members Billy Crystal and Valri Bromfield await their chance to speak with Lorne, who learns that Chevy has brought his fiancée Jacqueline (played by Kaia Gerber) to the proceedings.
As the cast and crew gather to rehearse their inaugural sketch, John and Gilda portray a couple, while Dan and Garrett take on the roles of robbers. During the performance, a mishap occurs when a set of lights crashes down, causing a small fire on the couch, which is promptly extinguished. Tensions rise as John nearly confronts a light technician, who ultimately quits after being criticized by the rest of the crew. Meanwhile, Lorne scrambles to replace the departing light technician while also being informed that he has over-scheduled sketches for the night, potentially jeopardizing their air time.
Garrett receives advice from Billy Preston (played by Jon Batiste) and his band, who encourage him to relax in front of the camera. Confiding in Gilda, he reveals his uncertainty about his selection among a group of comedians, given his background as a trained performer from Julliard. As conflicts arise, head writer Michael O’Donoghue clashes with network censor Joan Carbunkle regarding risqué jokes, prompting Michael to challenge Joan's faith. Furthermore, Jim Henson voices his frustrations about how the cast and crew handle his beloved Muppets.
Lorne seeks out his wife and co-writer Rosie Shuster, attempting to persuade John to don a bee costume for a sketch, which he finds beneath him. The couple engages in a deeper conversation about the show's potential and Rosie’s preference for crediting her maiden name versus “Michaels.” Later, Lorne and Dick meet with NBC executives to determine the fate of the show, aware that they may opt for a rerun of "The Tonight Show" if the first episode doesn’t impress. Tensions simmer as Dick encounters exec Dave Tebet, who expresses skepticism about "Saturday Night." Cheerful at first, Tebet’s demeanor changes when he becomes intrigued by Chevy, who showcases his abilities in front of the television executives.
As the chaos continues, Lorne is forced to mediate an altercation between a crew member and a delivery person amidst set construction. With Dick pressing Lorne to articulate the core concept of the show without a clear idea himself, Lorne finds his thoughts muddled. Meanwhile, host George Carlin and musical guest Janis Ian attempt to prepare, yet Carlin’s abrasive attitude leaves a sour taste among the team.
Tensions reach a breaking point when John and Chevy have a heated exchange in the dressing room, stemming from John's reluctance to shave his beard. After some persuasion from Lorne and Rosie, both men eventually resume their preparations. During a moment of frustration, Lorne receives a phone call from none other than Johnny Carson (voiced by Jeff Witzke), who subtly threatens Lorne’s show’s viability, knowing it conflicts with his own programming.
As Lorne witnesses John's unpredictable behavior, causing disturbances and trashing equipment, he fears for the success of their project. Meanwhile, Neil inadvertently becomes too intoxicated after trying to lighten the mood, leading to further complications in the studio. The arrival of legendary comedian Milton Berle for a sketch raises additional pressures, while Tebet warns Lorne of ongoing sound problems that could lead to cancellation.
In a frantic last-minute change, the cast pivots their sketches, creating humorous scenarios with Jane, Gilda, and Laraine on the construction workers sketch and Dan strutting in an outfit designed for comedic gawking. Chevy confronts Milton about his interaction with Jacqueline, only to be overwhelmed by Milton's confidence and bluntness. As Carlin grows exasperated, insisting on script changes, Lorne wrestles with keeping everything on schedule while juggling his team’s rising fears.
Trying to rally everyone, Lorne confronts his growing frustrations over cue cards and time allocations, but a moment of clarity emerges when Jim encourages him by reaffirming the passion behind his work. Lorne's resilience sparks a wave of energy among the cast and crew as they prepare for their impending live show.
As time races against the clock, Lorne encounters Billy and Valri bidding farewell, igniting a new determination in him. He collaborates with a struggling writer, Alan Zweibel, and enlists his help. Lorne then watches as Gilda marvels over John’s ice-skating antics, prompting discussions about their future success.
Finally, Lorne returns to the studio, reenergized and ready for the show. A young page, Finn Wolfhard, brings in a bustling audience just moments before they are to go live. Confronted by executive pressures yet again, Lorne discovers his defining message for the show: a reflection of what audiences can expect from a Saturday night in New York City.
With a clear vision, the cast springs into action, fixing technical malfunctions and preparing sketches on the fly. As the audience settles in and the clock approaches 11:30, tensions dissolve into excitement when Tebet greenlights them for a live broadcast. Kicking off their first sketch with gusto, John and Michael promptly garner raucous laughter. Then, as the moment culminates, Chevy steps onto the stage and delivers those iconic words for the first time: “Live from New York, it’s SATURDAY NIGHT!”
