If you’ve seen "Spy" with Melissa McCarthy, you’re already aware that the movie nails its first big laugh — the sneezing-assassin joke — within moments of the opening credits. Even if you know it’s coming, the timing is just right. And right away you think: There. Thank you. These people know what they’re doing.

How often does that thought run through your mind in a mainstream commercial comedy? Not often enough. It didn’t happen with "Ted 2," which may be a moderate box-office success, but it’s a weak, vaguely smelly sequel.

‘TRAINWRECK’ Rated: R Opens Friday

I bring up these movies in order to give "Trainwreck," written by and starring Amy Schumer, its full, brash, often riotous due.

At the risk of raising expectations, the first few scenes are among the best director Judd Apatow has ever done.

Schumer plays Amy, a fictionalized variation of the stand-up and "Inside Amy Schumer" Comedy Central personae that have carried Schumer to her current showbiz location.

The opening flashback sequence features Colin Quinn explaining to his two preteen daughters the futility and frustration of monogamy. Her childhood established in quick, deft expositional strokes — divorced parents, deceased mother, unrepentant horn-dog father afflicted with MS — we travel forward with Schumer’s Amy to the present. Her zesty, boozy, emotionally guarded love life includes more sex than love but she doesn’t mind. Does she?

At any rate, she does not like her men to sleep over. Our guide to Manhattan romance writes for a sub-Cosmo magazine called S’Nuff, edited by a ferociously egocentric boss played by a barely recognizable Tilda Swinton.

The plot of "Trainwreck" is simple: Amy is assigned to profile a successful sports medicine specialist, Aaron, played by Bill Hader. One abrupt but highly promising sleepover later, Aaron is convinced they should date. Amy resists. The movie cooks up some conflict to divide these lovers for a while, until about the two-thirds point, before Amy reckons with her more destructive and immature instincts.

The movie wouldn’t be much fun without them, of course. "Trainwreck" is all kinds of funny, and like any talent showcase worth its salt, the tone of the humor adjusts to suit the talents on screen.

Aaron’s friend and confidant is LeBron James, portraying a penny-pinching, "Downton Abbey" version of himself.

Brie Larson plays Amy’s married sister, whose agreeable low hum of a marriage (Mike Birbiglia plays her sweet husband) is everything Amy rejects. The sister relationship feels plausible and lived-in. So does the match of Schumer and Hader, both of whom take the opportunity afforded by "Trainwreck" to do the subtlest work of their careers.

Apatow generally has trouble with his wrap-ups, and the final third or so of "Trainwreck" feels longish and full of detours. Several scenes were asking to be cut. The climactic hookup scene, with Amy on the rebound with a magazine colleague, comes from a different film entirely.

The laughs in "Trainwreck" may come with an apology (the character describes herself as "broken"), but you believe the character’s transformation by romantic love, chiefly because Schumer and Hader are wonderful together. Gender inequity in the world of comedy deserves all the overdue attention it’s getting, and more.

Review by Michael Phillips, Chicago Tribune