BROOKLINE — It appears they saved the best for last.

The closing film for the Independent Film Festival of Boston, “World’s Greatest Dad” represents the work of a mature, intelligent, thoughtful director, a man who wants to discuss loneliness, pain and the strange environment of middle age.

Yes, I am talking about Bobcat Goldthwait. Bobcat “Police Academy” Goldthwait. It’s inexplicable. I knew Goldthwait as a decently funny comedian who specialized in gross-out comedy, and making Jimmy Kimmel marginally more palatable. I had no idea he was capable of this kind of comedy-stinging, rhythmic humor that makes your stomach hurt after laughing too hard.

I haven’t seen any other of Goldthwait’s films (the one he made prior to this, “Sleeping Dogs Lie” got excellent buzz at Sundance two years ago) but there’s tremendous prowess and skill in this film about a shlubby poetry teacher and his perverted son. I had my doubts the first 20 minutes or so. We are introduced to Williams’ character Lance, a man who calls himself a writer, though he’s never had anything published. He works as an unpopular poetry teacher at his son’s private school (before you ask, this is not “Dead Poet’s Society”) and soothes his ego by schtupping the much younger art teacher (Alexie Gilmore). His son Kyle (Daryl Sabara) is really more of a type of person than an actual one: he’s disrespectful, a liar, a pervert and unpleasant to everyone around him. It could make for some great verbal sparring between Williams and Sabara, but instead it’s just a slightly off-key, out of step round everyone calling each other a “fag.” A lot of these scenes were at least partially ad-libbed, and it’s extremely possible that Sabara simply got lost in William’s legendary improv.

But then, after a crucial turning point in the plot, something happens. Maybe Williams and his co-stars found some sort of equilibrium, or Goldthwait had a spark of inimitable genius, but I found myself laughing continuously for the next hour and a half. Without giving away a key point, I’ll say the film is about what happens when a person who dies is appropriated for someones own physical or emotional purposes (it’s not an accident that a band member from Nirvana makes a special guest appearance.) Williams is wonderful- amoral without losing our sympathies, sweet without being saccharine. Whenever he’s approached by someone he looks a little surprised that they noticed his existence. I’m not Williams’ biggest fan, but after this performance I absolutely forgive him for “Patch Adams.”

I wish I could have seen more at IFFB this year (note: do not work two jobs on top of covering a film festival. t’s bad for business.) But if I had to pick among the films I saw, “World’s Greatest Dad” is easily the best one I saw. Go on and see it if and when it gets distributed; a comedy for adults, real adults, is far too rare in these parts to ignore.