Rupert Pupkin (De Niro), a stage-door autograph hound, is an aspiring, mentally-deranged stand-up comedian unsuccessfully trying to launch his career. After meeting Jerry Langford (Lewis), a successful comedian and talk show host, Rupert believes his "big break" has finally come. He attempts to book a spot on the show but is continually rebuffed by Langford's staff and finally by Langford himself. Along the way, Rupert indulges in elaborate and obsessive fantasies in which he and Langford are colleagues and friends. Hoping to impress, Rupert invites a date, Rita, to accompany him when he decides to show up uninvited at Langford's country home. When Langford returns to his house from a golfing round, he finds Rupert and Rita settling in. Angered, he launches into a furious tirade against Rupert, telling him that his act is mediocre and that he's a lunatic who'll never amount to anything. While Jerry yells at him, Rupert continues trying to stay on his good graces, until an embarrassed Rita gets Rupert to finally leave.
When the straight approach does not work, Rupert hatches a kidnapping plot with the help of Masha (Sandra Bernhard), a fellow stalker similarly obsessed with Langford. As ransom, Rupert demands that he be given the opening spot on that evening's Jerry Langford Show (guest hosted by Tony Randall), and that the show be broadcast in normal fashion. The network brass, lawyers, and the FBI agree to his demands, with the understanding that Langford will be released once the show airs. Between the taping of the show and the broadcast, Masha has her "dream date" with Langford, who is duct-taped to a chair in her parents' Manhattan townhouse. Jerry convinces her to untie him and he manages to escape.
Rupert's stand-up routine is well received by the audience. In his act, he describes his troubled life (from growing up in a poor neighborhood with neglectful, alcoholic parents; to getting regularly bullied and beaten up during his adolescence) while simultaneously laughing at his circumstances. Rupert closes by confessing to the studio audience that he kidnapped Jerry Langford in order to break into show business. The audience laughs, believing it to be part of his act. Rupert responds by saying, "Tomorrow you'll know I wasn't kidding and you'll all think I'm crazy. But I figure it this way: better to be king for a night, than a schmuck for a lifetime."
The movie closes with a news report of Rupert's release from prison, set to a montage of storefronts stocking his "long awaited" autobiography, King For a Night. The report informs that Rupert still considers Jerry Langford his mentor and friend and that he and his agent are currently weighing several "attractive offers", including comedy tours and a film adaptation of his memoirs. The final scene shows Rupert taking the stage for an apparent TV special with a live audience and an announcer enthusiastically introducing and praising him, leaving the viewer to decide whether it is reality or Rupert's fantasy.
Who does Marsha have her dream date with?
Langford