Behind the gross-out humour and lewd romps of Trainwreck lies a very traditional rom-com. And a very traditional young lady.

Whisper it quietly because the Judd Apatow-helmed comedy, from the same school as Bridesmaids, attempts to disguise its prim roots beneath layers of brutal sexual encounters, crude girl talk and boozy nights out.

This is a romcom in the era of oversharing – Love, Gruesomely.

It is penned by its star – the versatile and confident big screen newcomer Amy Schumer – and tells a tale of an aimless New York girl living up to her errant father’s maxim – “monogamy isn’t realistic”.

Therefore this party girl (“pretty-ish, approachable”) has sex first, asks questions later, never spends a night with her conquest and always ensures that the cast of characters is in perpetual rotation. (The film credits are full of “One-Night Stand Guys”.)

This whirligig lifestyle is one in the eye for Smug Married Sister Kim (Brie Larson), who has bagged a hubbie and is therefore, to Amy, a symbol of unforgivable compromise.

Amy works for S’Nuff, a low-rent male magazine (Coverline: “You’re Not Gay – She’s Boring”) under the hard-as-nails editor Dianna, played with comic aplomb and an English estuary accent by Tilda Swinton.

The writer is dispatched into the world of sports – which she hates – to interview boringly nice knee doctor to the stars Aaron (Bill Hader).

Sports stars can therefore make legitimate cameo appearances to give the drag-along boyfriends something to enjoy. Turns out LeBron James is a good sport in all senses – and can act. (Also, oddly, Chris Evert appears with a potty-mouth for LOLs.)

Amy develops feelings for her doctor and is even more out of control than when she’s had one too many of the fizzy wines (which is most nights). But the moment romance strikes is the moment the film loses much of its hard-won, hard-edged originality.

Amy has a fear of intimacy – and men breathing on her

The film is full of sharp lines, good writing, busy characters and affecting sequences (when Amy pays tribute to her deadbeat dad she manages to destroy his character and make him adorable simultaneously).

And Amy, the writer, isn’t ashamed to make Amy the character a thoroughly tiresome, whiny and annoying bore who most men, who aren’t incredibly rich, dishy, loyal, doctorly and dreamy, would leave behind when the first free cab blew by.

Ultimately, and disappointingly, the film flinches in the face of its own manifesto. Spoiler alert – the answer to broken Amy’s woes isn’t within Amy. It’s about dumping her take-it-or-leave-it, elbows-out personality in favour of drone cheerleader conformity.

If the story had been written by a man, I suspect the female audience would cross their arms in fury and shout: “Typical sexist propaganda!”

As it is, they will probably laugh like drains at, and with, a notable new screwball talent.