There aren’t many things in this world as funny as simple, clueless sincerity. The TV series “The Office” gets this. “Cedar Rapids,” the new insurance salesman-convention comedy starring Ed Helms, may be “The Office” meets “The Hangover,” with plenty of lowdown and “Oh no they didn’t” laughs. But what makes it work is its footing in reality and the heartfelt naivete of its hero.

Helms is Tim Lippe, small-town insurance agent, midman on the totem pole at his Brown Valley, Wis., agency. He’s spent his whole working life there, convincing clients that “I’m gonna take care of you,” and always keeping his word. But just as reliably, he’s slighted by the boss (Stephen Root, a comic volcano here), always passed over for that coveted insurance agents’ convention in favor of the smarmy star salesman, Roger (Thomas Lennon).

But tragedy strikes and Roger’s death (it’s a funny one) means that Tim must get on an airplane, for the first time, fly to Cedar Rapids, Iowa, for the first time — to be away from home for the first time.

Tim leaves behind his lover, who happens to be his former sixth-grade teacher (Sigourney Weaver), straps on a money belt full of traveler’s checks and ventures into the big, wide world and the wild, wild big city — Cedar Rapids.

He has two missions: Win the coveted “two diamond” status from the insurance association president (Kurtwood Smith). And “avoid Dean Ziegler like the plague!”

Ziegler is trouble. Ziegler is unholy. He may even be unscrupulous. And he is played to gregarious, grotesque perfection by John C. Reilly. “Deanzie” is that blowhard who figures his loud voice, quickness with a crude quip or coarse nickname and his ability to hold his liquor make him the life of the party — any party. Naturally, Tim is forced to share a room with “Deanzie” and “an Afro-American” agent (Isiah Whitlock Jr., deadpan, in on the joke and a hoot).

Tim is quickly caught up in a whirl of intrigue, back-stabbing and hard partying, just another soul pulled into the vortex of Deanzie. The lure of Deanzie is temptation itself, temptation in its many forms. It’s not just the booze and general rule-flouting. There’s also the tempting, flirtatious and crude Joan (Anne Heche), an old hand at these leave-home/cut-loose conventions, somebody who knows “What happens in Cedar Rapids stays in” you-know-where.

Director Miguel Arteta (“Youth in Revolt”), working from a Phil Johnston script, makes this a movie with way-out moments that are never that way out. It’s a farce with sexual come-ons and actual sex — the Boy Scout Tim’s first encounter with a hooker and a crack pipe — but “Cedar Rapids” never loses track of the humanity of its characters, screwballs, one and all, but sometimes warm people who never cross over into caricature.

Less successful is the film’s broad swipe at conservative Midwestern hypocrisy, at seemingly sainted “small businessmen” (Smith’s president character is nicknamed “Pope”) who trot out the trimmings of faith and country, but who are profane and corrupt to the core.

Heche lets us sense Joan’s resignation to a dull, depressing life in “Oh my god” (Omaha) Nebraska. Reilly never lets Deanzie turn so gonzo that we don’t see the divorced-man loneliness Deanzie’s bonhomie hides.

And Helms keeps Tim’s “gee whiz” innocence just this side of reality. He’s unworldly and a bit of a Pollyanna. But listen to him talk about that first childhood encounter with an insurance agent, a man whose job it was to “get people’s lives back on track,” and you too will think, “Yeah, they really are heroes.” And laugh.