While driving through Beechum County, Alabama, two New Yorkers, Bill Gambini and his friend Stan Rothenstein, find themselves in a surprising predicament. They accidentally overlook paying for a can of tuna at a convenience store, and shortly after their departure, the store clerk is tragically shot dead. The duo, matching the description of the culprits provided by witnesses, is promptly pulled over by the local sheriff, Farley, and taken into custody regarding the murder.
The situation escalates as Billy, believing they are merely being detained for shoplifting, hastily declares that he shot the clerk — a statement that is misinterpreted as a confession, leading to murder charges against him, while Stan faces accessory charges. In a desperate bid for help, they reach out to Billy's mother, who informs him about a family attorney, Vincent LaGuardia "Vinny" Gambini, portrayed by Joe Pesci, who travels to Beechum County with his fiancée, Mona Lisa Vito played by Marisa Tomei. However, Vinny, a Brooklyn personal injury lawyer who has just passed the bar after numerous attempts, lacks trial experience.
Despite Vinny’s determination, his lack of familiarity with courtroom procedures and his brash demeanor put him at odds with the stoic judge, Chamberlain Haller. During initial proceedings, Vinny attempts to negotiate on behalf of his clients without a trial but finds himself charged with contempt due to his inappropriate attire and unkempt appearance. Compounding their problems, he fails to effectively question key witnesses, giving the impression that the district attorney, Jim Trotter III, has an unbeatable case.
As frustration mounts, Stan considers replacing Vinny with public defender John Gibbons. Yet, Billy decides to give his cousin one last opportunity. Vinny soon realizes that Trotter’s case relies heavily on circumstantial evidence. Utilizing a card trick as an analogy, he reveals to Billy the underlying weaknesses in the prosecution's claims.
When the trial finally begins, Vinny confronts numerous challenges, including a wardrobe mishap that causes him to appear in a gaudy tuxedo. Nevertheless, he manages to turn the tide with his energetic questioning style, successfully dismantling witness testimonies. His persistence pays off when he demonstrates the uncertainties in the accounts of witnesses, including a bewildered man who recalls cooking grits during the alleged crime.
As the courtroom drama unfolds, Vinny faces unexpected hurdles, including surprise witness testimony from an FBI expert linking the tire markings from the scene to Billy’s vehicle. In a moment of clarity, he realizes that he needs Lisa's expertise in automobiles to counter the expert's claims. Lisa successfully testifies that the car responsible for the tire marks could not have been a 1964 Buick Skylark, but rather a 1963 Pontiac Tempest.
Vinny’s strategic questioning of the sheriff reveals that a recently apprehended duo, resembling Billy and Stan, had been caught driving that very stolen Pontiac, armed with the same caliber weapon used in the clerk's murder. Ultimately, Trotter concedes and moves to dismiss all charges against the boys.
Throughout this intricate-plot legal battle, Vinny and Judge Haller engage in an elaborate play of wit and intellect as Haller gradually uncovers Vinny's dubious claims of legal experience. Eventually, with the support of Lisa and a series of clever legal maneuvers, Vinny earns the judge's respect, leading to a heartfelt conclusion where Haller acknowledges Vinny's talents.
The film wraps up with Vinny and Lisa driving away, humorously bickering about their impending wedding plans, showcasing the joyous but tumultuous journey they’ve shared.
