It might be the most accessible film comedian Bobcat Goldthwait has yet made, but “World’s Greatest Dad” will still seem mighty strange to most viewers. It’s a very dark comedy-drama about a milquetoast high school teacher and his absolutely repulsive teen son, who becomes a cultural icon after a terrible accident. And though it suffers some stumbles along the way and definitely chickens out in the end, for much of the running-time it’s a deliciously deadpan satire on the cults that grow up around dead celebrities.

Robin Williams, in his ultra-recessive mode, is Lance Clayton, a forlorn English teacher at a Seattle campus. An aspiring writer, his submissions have been rejected by publisher after publisher, and his romance with fellow teacher Claire (Alexie Gilmore) never gets past the coy affection stage, and seems positively threatened by studly new English instructor Mike (Henry Simmons), whose creative writing class proves enormously popular while Lance’s poetry class attracts nobody but a few misfits. To make matters worse, Mike’s first article is accepted by The New Yorker.

But it’s Lance’s home life that’s the real disaster. His son Kyle (Daryl Sabara) is a totally obnoxious kid, whose grades are lousy and only passion seems to be the grossest sort of pornography, and who’s understandably loathed by all his classmates save dorky pal Andrew (Evan Martin)—and even he’s nonplussed by Kyle’s constant rudeness and general piggishness. The kid spies on an elderly neighbor as she undresses at night, and when Lance forces him to go to dinner with him and Claire, Kyle uses his cell phone to take some revealing under-the-table photos of her. And the kid is adept at pushing his dad around mercilessly, even though he’s at the point of being sent to an “alternative” school.

The plot takes a major turn about half-way in, when Kyle’s habit of solitary self-gratification comes to an unhappy end and his father uses his literary skill to cover the truth up. His efforts bring Kyle posthumous adulation and himself the literary recognition he’d always wanted—though mixed with pain. Had the picture ended on this bleak note, it would have been a delightfully macabre take on society’s skewed obsession with celebrity and the idiotic idolatry that results from it. Unfortunately, Goldthwait opts for a last-minute reversal that values sweetness and integrity over deception and nastiness. Perhaps he felt it necessary to lift the viewer’s spirits and tell them that one can still make the right choices. But even then, the way he chooses to portray Lance’s liberation—especially in a swimming-pool scene—seems all wrong (though there’s a nice last-minute joke involving Andrew).

Still, disappointment that Goldthwait didn’t see his vision through is largely mitigated by the good stuff that’s gone before. The performances are for the most part acceptable if not outstanding (Goldthwait himself does a cameo, as does singer Bruce Hornsby in an amusing gag), with Sabara coming across as genuinely disgusting. But of course it’s Williams who has to carry the picture, and he largely does so with the sort of controlled turn familiar from his work in films like “One Hour Photo,” though there are a couple of moments when he cuts loose, with mixed results.

From the technical perspective “Dad” is pretty plain, but the visual simplicity suits most of the story (though the “celebrity” segments toward the close could have used more pizzazz).

“World’s Greatest Dad” joins such bizarrely dark pictures as Bob Balaban’s “Parents” in a small but select group that will probably speak to a small audience, but an appreciative one.