The self-reflectivity of The Errand Boy is so pervasive it truly becomes the content of the film, and alone justifies Lewis's reputation as a master of postmodern sentiment. In the prologue, the narrator gushes over the magic of Hollywood and the diverse genres it's defined and perfected ("westerns, cheesecake, suspense, brutality, how 'bout a love story?"). But immediately, the narrator turns around and admits that Hollywood will take you anywhere you want to go, except behind its own façade. At which point the example of "brutality," a woman being slapped repeatedly, is revealed to be a stunt with a pro wrestler in drag standing in for the woman. And the paramours of the love story are revealed, behind the scenes, to be a genuinely married pair of actors who, naturally, have a pretty "brutal" relationship themselves. (It's up for grabs whether the subversive cut from their quarrelling to the water town above Desilu Studios was an intentional reference to Lucy and Desi's relationship or not.)
The chairman and CEO of "Paramutual Pictures" is perplexed as to how his studio could possibly be losing money, especially since most of the pictures they release are hits. His solution is to hire someone to spy on the productions on his lot, but someone too stupid to realize he's eavesdropping. They decide on the boy sloppily pasting up a billboard poster for the next Jerry Lewis movie (which we are presumably watching), Morty S. Tashman (Lewis). As with most of Lewis's films, those expecting the setup to find a direct answer in the film's outcome will be sorely disappointed. The remainder of the film mostly consists of Tashman wandering from set to set, from script girls' office to costume shop to studio commissary to Hollywood premiere (where he inadvertently becomes arm candy for a breathy starlet whose resemblance to Marilyn Monroe certainly adds another layer of intrigue, given Lewis's recent claims to have banged JFK's lover), ostensibly there to spy, but far too concerned with not destroying everything around him to really pay any attention to possible economic imprudence.
The scene in the commissary is a great example of what Chris Fujiwara wrote was "Lewis's willingness to let a scene's formal properties determine its unfolding, and it demonstrates his use of duration and repetition as sources of humour." Three children show up at his candy counter, and the first requests a quarter worth of jellybeans, which are stored in a gargantuan glass vase on the highest shelf, requiring Lewis to climb a rickety ladder (on wheels, even) to retrieve them. Since he returns the vase to its place on the shelf between kids, and all three want the same thing, the joke really isn't that he has to climb the ladder time and again, but rather that the sheer length of the gag has extended far beyond the initial humorous climax (where most comedians would move onto the next gag).
When Tashman, later in the film, tries to clock out for the day at Paramutual's front gate, he finds that the time clock refuses to punch his card. But of course Lewis can't clock out of his own movie, which revolves obsessively around his examinations of his own status as a filmmaker and as a performer. Earlier, I suggested that The Errand Boy's setup doesn't resolve itself in a forthright fashion, which isn't exactly the final truth. In the end, Lewis has cheekily suggested that the reason Paramutual can't seem to make their financial ends meet is because they're foolish enough to hire volatile people like himself, groom them to be stars and let them run untethered on their backlots.
In that, the film is a revolutionary call to arms, a challenge to everyone in the system to remain true to their own radical spirit. However, Lewis is too aware of his own hypocrisies to presume himself. The film ends with Tashman, now a successful Hollywood bigshot, showing the new poster boy the art of gluing billboards, and as it turns out the new guy is also Jerry Lewis, suggesting that no matter how unique and brazen anyone in Hollywood might consider themselves, they've always depended on learning the lessons of their mentors (which is probably why Lewis's character's name sounds so naggingly like Tashlin). And there will always be someone down the line who will steal your schtick.