This is tricky: How to talk about “World’s Greatest Dad,” the latest comedy from Bobcat Goldthwait, without giving away all its twists and twisted details?

It’s extremely dark and daring and definitely not for everyone, but it shows that with his third film as writer and director, Goldthwait is honing a unique and fearless voice, and that’s exciting to see.

The comic’s first, 1991’s “Shakes the Clown” (in which he also starred), was about an alcoholic party clown; his second, 2006’s “Sleeping Dogs Lie,” was about a woman who enjoys a sexual dalliance with her pet. This time, Goldthwait explores the ugliest and most selfish human instincts after the death of a teen.

But what can we tell you about it?

For starters, Robin Williams stars as Lance Clayton, a loser of a high school poetry teacher. He had dreamed of fame and fortune as a novelist; instead, he can only get a handful of students to sign up for his elective course, while Mike (Henry Simmons), the handsome and popular creative writing teacher, finds his classroom packed. He’s enjoying a romance with Claire (Alexie Gilmore), the perky and much younger art teacher, but she doesn’t want anyone to know about it and frequently cancels dates.

Lance’s 15-year-old son, Kyle (Daryl Sabara, “Spy Kids”), whom he’s raising alone after a divorce, is among the students who view him with disdain; then again, Kyle is a vile human being. All he cares about are video games and graphic porn. He talks a lot of misogynistic, homophobic trash as part of his nerdy bravado, and he bullies the only friend he’s got, the scrawny Andrew (Evan Martin).

The uncomfortable discussions Lance and Kyle have about sex and drugs provide some of the film’s earliest cringe-inducing laughs, and the movie’s low-budget aesthetic seems to fit the coarse dialogue. But then a freak accident alters the way both are perceived on campus, a social shift that Lance exploits in hideous ways. And that’s about all we can say about that.

Perhaps because he and Williams are such good friends, Goldthwait makes good use of the creepier elements in Williams’ on-screen personality, which we’ve seen only rarely in movies like “One Hour Photo.” The character also provides an amusing little play on his inspirational instructor roles in “Good Will Hunting” and “Dead Poets Society.

“World’s Greatest Dad” borrows maybe a bit too obviously from the satirical classic “Heathers” in its skewering of hysterical posthumous hero worship. And the movie essentially relies on the same joke being told over and over. But Goldthwait finds enough clever ways into that joke to make it seem fresh, and he makes you curious to see how far he’s willing to push it.

You have no idea.