Although it may not have the year's most original premise, amnesia being a plot twist as old as — well, I forget — "Samantha Who?" is as perfectly realized a comedy as the fall has to offer.

Starring Christina Applegate as a woman who awakens from an eight-day coma with no memory of the horrible, if successful, person she recently was, it sits companionably among a number of other fanciful ABC series hitched to strong female leads — "Ugly Betty," "Grey's Anatomy"/"Private Practice" and "Men in Trees" among them.

It is sweet and smart, without one term compromising the other, fast in an old-fashioned way: Six or seven decades ago the idea might have served Irene Dunne or Mitchell Leisen well. But it is not a studied pastiche — it's the real thing, a contemporary screwball comedy in which the verbal asides and half-concealed looks carry as much comic charge as the waving arms, double-takes and load-bearing jokes, and the minor characters are as lovingly realized as the major.

Thematically, it bears a passing resemblance to "My Name Is Earl" — someone who was bad turns good, after being hit by a car — although the milieu is a few tax brackets higher, the mechanics of the conversion different and the energy level considerably more energetic. Still, each is a comedy of better nature versus worse. And like Earl, new good Sam is horrified to realize what an evil Samantha she formerly had been: Picking up a message on her boyfriend's phone she hears someone say, "Whoever tried to run Sam down, we're gonna find that guy ... and we're gonna buy him a drink."

Her challenge now is not to find her way back to her former self but to learn how to function within her old life without messing up. She keeps her amnesia a secret at work, a real-estate development company ruled by Rick Hoffman (the bad magician friend from "Jake in Progress").

"How am I supposed to start fresh when my past just keeps reaching into my future and pulling into my present?" Samantha wonders.

This is the best use I've ever seen made of Applegate, whose air-headed Kelly Bundy still haunts the airwaves a decade after "Married ... With Children" was laid to rest. On the verge of 36, Applegate has a quizzical child's face that sits not quite incongruously on her hot bod, and this serves Samantha well: fully grown and brand-new. But she's deft at playing her flashback-glimpsed awful earlier self as well.

The supporting cast is just as good; they all know how to keep a volley going. Jennifer Esposito plays best friend Andrea, a red-dressed devil tempting her back to the dark side. When Samantha goes off on a hapless bystander, Andrea cries happily, "My baby's still in there!"

Melissa McCarthy is a less fabulous older friend who writes herself into Sam's unremembered history. ("We aren't that close now," she is forced to admit, "but we were up until seventh grade.")

Also spinning the past is Jean Smart — another actress of greater gifts and depth than television always makes apparent — as Samantha's mother, Regina. Although many of her lines turn on her self-involvement, Smart plays the whole person — sad and hopeful, loving a daughter she doesn't like.

"You made me who I am," says Sam, moving back home to learn about herself.

"That is a terrible thing to say," says Regina.

But this is just where it begins.