Rupert Pupkin (Robert De Niro) is an aspiring, mentally-deranged stand-up comedian unsuccessfully trying to launch his career. After meeting Jerry Langford (Jerry Lewis), a successful comedian and talk show host, Rupert believes his "big break" has finally come. He attempts to book a spot on Langford's show, but is continually rebuffed by his 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 shows up uninvited at Langford's country home. When Langford returns home to find Rupert and Rita settling in, he angrily tells Rupert to leave. While Jerry yells at him, Rupert continues trying to stay in 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 episode of Langford's 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. Langford 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 upbringing while simultaneously laughing at his circumstances. Rupert closes by confessing to the studio audience that he kidnapped 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. Rupert still considers Langford his friend and that he is 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 him, leaving the viewer to decide whether it is reality or Rupert's fantasy.
