Rupert Pupkin, a stage-door autograph hound, is an aspiring stand-up comic with obsessive ambition far in excess of any actual talent. A chance meeting with Jerry Langford, a famous comedian and talk show host, leads Rupert to believe that his "big break" has finally come. His attempts to get a place on the show are continually rebuffed by Langford's staff and, finally, by Langford himself.
Along the way, Rupert indulges in elaborate and obsessive fantasies where he and Langford are colleagues and friends.
When the straight approach does not work, Rupert hatches a kidnapping plot with the help of Masha, a stalker familiar with Langford's movements. As ransom, Rupert demands that the kidnapping be kept secret, that he be given the opening spot on that evening's Jerry Langford Show, and that the show be broadcast in normal fashion. The network brass, lawyers, and the FBI agree, with the understanding that Langford will be released once the show airs nationally. Between the taping of the show and the national broadcast, Masha has her "dream date" with Langford, who is duct-taped to a chair in her parents' Manhattan townhouse.
Rupert's stand-up routine is well received. He closes by confessing to the audience that he kidnapped Jerry Langford in order to break into show business. The studio audience laughs, thinking that it's a 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 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."
The final scene shows Rupert taking the stage for an apparent TV special with a live audience an announcer enthusiastically introducing and praising him.