T. Sean Shannon won an Emmy as a writer for “Saturday Night Live,” and with “Harold,” his first feature as a writer and director, one can see why: Mr. Shannon is the rare comedy writer who doesn’t just envision the punch line, but the nuanced path a joke can take to get there. But though he may be inventive and gifted as a writer, what’s clear from this erratic film, which opens in the city on Friday, is that he’s not quite ready to don the director’s hat.

The layout is shaky, the editing incompetent, and the static visual style uninspired, but the raw concept behind “Harold” is just bizarre enough to beguile. Harold (Spencer Breslin) is a prematurely balding teenager who dresses, talks, and acts the part of an adult science-fiction nerd. He slumps his shoulders as he shuffles along, greeting everyone he meets at his new high school with a dose of impish politeness. He’s a middle-age library regular trapped in a world of popular girls and bullies, struggling to make the best of things even when he’s being doused with milk in the cafeteria by his infantile peers.

Harold’s life is one of perseverance, and young Mr. Breslin makes his awkward social skills both a humiliating scarlet letter and a badge of honor. Pick on him as they might, the rude kids in Harold’s school just won’t get him down. Instead, he pushes his glasses up on his nose, ducks his head, and stubbornly refuses to give up. Imagine Napoleon Dynamite with a higher I.Q.

Harold may be lovably pathetic — or pathetically lovable — but there are scenes in which Mr. Shannon miscalculates the cruelty factor and drags his story down into pure heartbreak. Harold is so painfully out of place and seemingly doomed to a life of misery that the humor at his expense becomes the only variety on offer. No surprise, then, that the film finds its surest footing when its protagonist finally tries to break through the barriers of his stratified high school existence.

Without a sense of trepidation, Harold sets out to hit on the popular girl in school, bravely mocks the bully who has made it a mission to humiliate baldy, and even attempts to intimidate the jock who is trying to pressure Harold’s sister into sleeping with him.

Harold’s focus becomes directed outward at the cruel world, and the conceit of a hairless loser throwing caution to the wind produces its share of funny moments. But as the movie progresses and Harold shrinks further into his shell, becoming ensconced in his own state of denial, the jokes fall short as events grow more disturbing. With the introduction of an intriguing classmate, Rhonda (an effervescent Nikki Blonsky), one senses a potential out for Mr. Shannon. But Rhonda is dreadfully underwritten, and Harold is so dismissive of her kindness that her involvement in the story becomes less a source of redemption than a slippery slope to Harold’s uglier side.

In fact, much the same can be said for the supporting characters here. Cuba Gooding Jr. plays the carefree, semi-creepy janitor who doesn’t really help Harold grow so much as give him someone to interact with. And a gaggle of “SNL” alums pop up — Colin Quinn as a strip-club regular, Chris Parnell as the high school gym teacher, Rachel Dratch as another teacher, etc. — to provide filler so Mr. Shannon can stretch out his story.

As played by Mr. Breslin, Harold is an inspired creation, at times a fidgety Woody Allen, at others a brewing George Costanza. He’s a funny freak show of a teenager, unlike any we’ve seen on the big screen, but he’s left stranded by the absence of credible outside forces needed to lend him a sense of depth. At some point, there’s no choice but to feel sorry for the boy, and pity, as surely Mr. Shannon knows, is not funny.

ssnyder@nysun.com