Neil Simon has been writing plays for 30 years and he still can't handle the basic elements of dramaturgy. "Lost in Yonkers," which opened last night at the National Theatre, begins with a scene between Jay and Arty, two teenage brothers sitting in their grandmother's hot living room in 1952 Yonkers talking and talking and talking. It's like one of those expository conversations between the maid and the butler in theatrical chestnuts of yore: "Lord So-and-so looked poorly last night." "Yes, I'm afraid it's his gout. You see ..." and then so on in infinite detail about the gout. Simon goes on in infinite detail about why the boys are sitting there, their family history (dead mother) and the personalities of three characters yet to arrive onstage: Grandma Kurnitz, Aunt Bella and Uncle Louie. During all this the boys' father, Eddie, pops nervously in and out of the room. After roughly 25 minutes, Jay announces, "There's something going on here." But it's another quarter-hour before Grandma enters and something finally does begin to go on.

Simon also has trouble with that other generally essential dramatic element, story. "Lost in Yonkers" doesn't have one. It does have a setup: In debt to a loan shark, Eddie leaves New York to earn money, depositing his sons with their fierce grandmother. After this, nothing actually happens. The emotionally childlike Aunt Bella is going to get married, then she isn't. Second-rate gangster Uncle Louie is in danger, then he isn't. Grandma is a monster, then she isn't. Simon can't seem to sort out his attitudes about her. He suggests she destroyed her children, then shows that the children turn out all right. He presents her as a dreadful women, then decides she's had her reasons. About the only thing he's sure of is that, like everyone else in the play, she's good for a laugh.

Simon's serious dramas have always been denatured by his unwillingness to pose any problem that can't be fixed with a gag. This is what made his autobiographical trilogy ("Brighton Beach Memoirs," "Biloxi Blues," "Broadway Bound") squishy in the middle. It's what sometimes gives a crass, desperate edge to his humor (in an audience-grabbing lunge, "Biloxi Blues" opens with a fart joke). And it's what keeps "Lost in Yonkers" away from the troubling adult emotions Simon seems to want to deal with. This is a play with intimations of child abuse throughout, yet in the end, no real harm has been done; everything is as fine as if the Kurnitzes were a TV family from the '50s.

You can't say there's nothing going on onstage, though: There's some smashing acting. Gene Saks has not only kept the non-action snapping along, he's cast the play with brilliant shrewdness. In the small roles, Jamie Marsh and Danny Gerard are wary and resilient as the two boys, and Lauren Klein manages to make something effective out of Gert, whom Simon has saddled with what he appears to regard as a comic breathing problem.

In the most important of the larger roles, Irene Worth plays the grandmother with a soul-deep bitterness no laughs can dispel. Mark Blum makes the weak Eddie sympathetic and gives him his own small core of strength. He's particularly moving in his frazzled, hysterical, heartbroken confession to his sons about his debt. Bella, the victimized crazy, is the kind of part that leads actresses to paroxysms of fragile charm and trembling self-pity. Mercedes Ruehl, tall, tough and funny, smacks the pathos right out of the role. Her Bella may be sweetly optimistic in her feeble-mindedness, but she's also, within her limits, a survivor.

As Uncle Louie, Kevin Spacey makes a Cagney-imitator role genuinely menacing. Spacey has a rare actor's gift: He can project danger. You're never sure of him; he might do anything. All by himself, Spacey injects flaccid scenes with tension. When he's paired with Worth or Ruehl, the encounters can be electric. "Whatever I've accomplished in my life," he tells his mother when she berates him for his criminality, "you're my partner" -- and he throws her a smacking kiss as contemptuous as spit in the face. For a moment, the stage is alive with real hate.

But then Louie leaves and does an offstage transformation into a war hero. What stops Simon from following his darker impulses to an honest conclusion? It's not that he's insulated or naive. "Anger has been in me for a long time," Grandma Kurnitz says grimly at one point, and there are indications it's in Simon too. It's a cliche that the humorist creates his jokes from rage, and in the past decade stand-up comics have often substituted rage and its shock for the joke itself. Simon does the opposite -- he softens his fury and pain as if, finally, he didn't want to bother us with them.

"Don't pay me for being born," Grandma says, rejecting a birthday gift. "I been paid enough." If he could write that line, Simon's probably been paid enough by life too. Maybe someday he'll write a play about it.

Lost in Yonkers, by Neil Simon. Directed by Gene Saks. Set and costumes, Santo Loquasto; lighting, Tharon Musser. With Irene Worth, Mercedes Ruehl, Kevin Spacey, Mark Blum, Lauren Klein, Jamie Marsh and Danny Gerard. At the National Theatre through Feb. 10.