Kate, a successful businesswoman, wants the baby she never had time for, but her body's not cooperating.

Angie, not very successful at anything, really, can have a baby — and gets herself hired out by a snooty, spacey surrogate agency to have Kate's.

That, in a nutshell, is "Baby Mama." There is a somewhat satisfying dramatic twist somewhere around the middle, but for the most part the film is as predictable as that brief description makes it sound.

It's also hilarious, laugh-out-loud funny. That it's more a series of sketches, some of which are revisited throughout the film, isn't necessarily a slight; the same is true of movies like "Animal House" and "Caddyshack," and you don't hear anyone complaining about their lack of plot all these years later.

That this is the structure of the film is also not a surprise, as it's a vehicle for several "Saturday Night Live" veterans, including its stars — Tina Fey, who plays Kate, and Amy Poehler, who plays Angie.

Fey, who created and stars in NBC's "30 Rock" — the funniest show on television — is best when she's the overly rational calm in the middle of a storm of unhinged characters. And while she neither wrote nor directed "Baby Mama" — Michael McCullers, another "Saturday Night Live" veteran, handled both chores — that's where she finds herself here.

Kate is the vice president of an organic food company founded by a New Age nut case, played with gusto by Steve Martin. Her T-shaped uterus won't allow her to conceive, so she visits a surrogate agency founded by Chaffee Bicknell, an unusually fertile older woman (Sigourney Weaver, also top-notch).

The agency helps Kate find Angie, who "discontinued high school" and is stuck with a low-rent, money-grubbing common-law husband named Carl (Dax Shepherd, whose first role, as "vomiter at party" in 1998's "Hairshirt," is fitting, in a good way).

Angie and Kate's lifestyles clash, particularly after Angie moves into Kate's apartment. To say that each teaches the other important lessons about what's missing in their lives again makes "Baby Mama" sound trite, and maybe it is. Who cares? It's too funny for that to matter.

Fey and Poehler are terrific together, their relationship the heart of the film. "Baby Mama" is at its best when they are in Kate's apartment, arguing, fighting, bonding, whatever. Some of the throwaway jokes, the little asides in their scrappy banter, are the funniest things about the movie. Their delivery — Poehler's in particular — makes even the most sitcom-ish situations genuinely funny.

When they're joined by Oscar (Romany Malco), Kate's friendly doorman, it's even better.

Greg Kinnear shows up as Rob, the bemused owner of the Super Fruity smoothie store in the neighborhood Kate chooses for her company's next store. Yes, their relationship is also predictable. But he's funny, too.

So it goes with "Baby Mama." The humor outweighs the holes in the story, Fey and Poehler look to be a comedy team built to last — let's hope so — and those who find Judd Apatow's comedies lacking in decent female roles (present company excluded; see "Forgetting Sarah Marshall" for evidence to the contrary) have found their film.

A successful delivery all the way 'round.

Bill Goodykoontz is the film critic for The Arizona Republic and chief film critic for Gannett. Reach him at bill.goodykoontz@arizonarepublic.com. Read his blog at http://goodyblog.azcentral.com.