Everything you've heard about "Knocked Up" is true: It's even more riotously and consistently hilarious than its predecessor, Judd Apatow's 2005 sleeper hit "The 40-Year-Old Virgin," with even greater heart. No small feat.

As writer and director, Apatow has the rare ability to find just the right tone all the time — a bold yet delicate balance of humor that's raunchy without being mean, sweet without being saccharine. "Knocked Up" also features a cavalcade of pop culture references that never feel gratuitous, as well as some in-jokes that never seem smug.

Here's how good Apatow is: He even manages to make Ryan Seacrest funny, in one spectacular scene in which the "American Idol" host takes a sledgehammer to his self-important image.

Seth Rogen, who we already knew was funny from his scene-stealing performance in "Virgin," emerges as an unlikely but likable leading man as Ben Stone, a goofball of a slacker-stoner who enjoys a drunken romp with Alison Scott ( Katherine Heigl from "Grey's Anatomy" showing an unexpected knack for comedy), an up-and-coming entertainment reporter who's way out of his league.

Eight weeks later, she realizes she's pregnant — and, to borrow from Madonna, she's made up her mind, she's keeping her baby — forcing both of them to make major changes in their lives.

(The fact that Apatow approaches the idea of abortion and lets his characters ponder it before Alison chooses to have the child, without entering into any sort of heavy-handed moral or political debate, is just one more example of his assured touch.)

Clearly these people are totally wrong for each other. Ben is round and hairy; Alison is leggy and blonde. He sits around all day with his equally unmotivated buddies, working on a Web site that lists the exact moment when actresses get naked in movies; she wakes up early, spends time with her family and takes her job seriously at E! Entertainment Television.

But they must get to know each other, first as friends and then possibly as something deeper, if they're going to make this work. It doesn't exactly help that their primary example is Alison's older sister Debbie (Apatow's radiant wife, Leslie Mann) and her husband, Pete ( Paul Rudd, always a subtle comic talent), who don't have much in common anymore besides their two kids.

On Ben's side, there's even less help. The friends with whom he shares a tract house in the San Fernando Valley (Jason Segel, Jonah Hill, Martin Starr and Jay Baruchel, all old pals from "Virgin" and Apatow's short-lived TV shows "Freaks and Geeks" and "Undeclared") say they support him but they'd have no idea what to do with a baby, and they're too high all the time to figure it out.

This description, of course, makes these guys sounds useless and paints them into a corner as overly familiar types. But one of the best aspects of Apatow's script is that he makes his characters feel incredibly real. These are people who love movies and television and pop culture, and their analytical discussions of these topics always sound completely genuine. You may not get all the jokes — references to "Murderball" and Matisyahu play differently in different cities — but you'll laugh hard enough and often enough to want to go back a second time and catch the lines you missed.

When they tool on each other — which they do constantly — you know it's with affection, and Apatow obviously has great affection for them, too. Much of their banter was improvised, and Heigl, in a rare leading film role, steps confidently and easily into this male-dork-dominated society. It would be easy to write her off as eye candy, playing the straight woman amid a bevy of actors who get far showier roles, but she very much holds her own.

"Knocked Up" starts out with a one-night stand, but in the end it turns out to be a movie about loyalty. And except for being a bit on the long side — which you won't mind because you'll be having too much fun — it's pretty close to perfect.

"Knocked Up," a Universal Pictures release, is rated R for sexual content, drug use and language. Running time: 129 minutes. Four stars out of four.