“BLOCKERS”

***

(R, 1:42)

If you want to know what a culture thinks it thinks, watch drama. But if you want to know how it really thinks, watch comedy. Watch, for example, “Blockers,” which is exuberant in its crudeness and coarseness. It’s where comedy is now, and it’s very funny.

The premise is simple, and yet difficult to pull off. Three parents get wind of their daughters’ pact to lose their virginity on prom night. And so, they go off on a mission to stop them, setting in motion a one-wacky-night plot that gets more and more insane as it goes along.

What makes the premise hard to pull off is navigating the mores and morality. If the parents succeed in stopping the night, they also stop the comedy by reigning in anarchy. But a movie celebrating three girls having sex for the first time because they’ve made some kind of pact is too cold-bloodedly amoral for an audience to accept without hesitation. So “Blockers” walks that fine line, and it’s a tribute to the skill of Brian and Jim Kehoe’s script that, in the moment, that line doesn’t seem fine at all.

Kayla (Geraldine Viswanathan), Julie (Kathryn Newton) and Sam (Gideon Adlon) are best friends since childhood, and the naturalness of their interaction and the matter-of-fact, unvarnished presentation of their personalities indicate the skilled hand of director Kay Cannon (“Pitch Perfect”). Julie has a boyfriend she’s crazy about; Kayla has a guy she’s willing to get crazy with, and Sam is ambivalent about the whole thing, but wants to get this sexual milestone over with.

Their parents suspect nothing, until they interpret the emojis on their text message feed, and this triggers an almost irrational response in them. What’s good — the mark of strong comic writing — is that the response is different for each. For Lisa (Leslie Mann), it brings up all the trauma of becoming pregnant as a very young woman and having to abandon her career to become a single mother. For Mitchell (a very funny John Cena), it triggers a panic that his daughter is no longer a child — and also anger, because he can’t stand the smirky looks of her prom date. Meanwhile, Hunter (Ike Barinholtz) just feels protective, because he doesn’t believe his daughter really wants to be part of this pact.

The trick of a movie like this is to keep devising funny incidents. It’s as if the screenwriters, having crafted a situation, then must generate a series of five and 10-minute comic shorts arising from the set-up. And they do it — really well. Not every joke is a home run, but the jokes keep coming, and more than a few times, they’re hilarious.

There is a throwing up scene, for example, that’s not quite in the exalted league of the diarrhea scene in “Bridesmaids,” but it’s close enough. There is another very funny scene in which a 40-ounce bottle of beer is poured, by funnel, directly into the anus of one of the parents. I bring these scenes up, in particular, by way of a promise and a warning: If you don’t find this sort of thing funny, you might want to wait for the next drawing room comedy.

Not all of the humor is quite this gross. Leslie Mann has a mini tour de force in a scene in which she accidentally ends up underneath the bed in her daughter’s hotel room. The slapstick comedy of her efforts to escape is worthy of Lucille Ball.

Underlying “Blockers” is just enough seriousness that it connects as true. It’s about the awkwardness of that moment between childhood and adulthood, where the young person has to make his or her own mistakes, and the parents can only watch in horrified impotence, because mistakes will be made. That’s guaranteed.

At the same time, there’s a suggestion of a more European sensibility. At one point, Kayla (Viswanathan) asks her father (Cena), “Why is sex so bad?” And for a moment he sits with that question in a confused silence.