For three decades, the Ensemble Studio Theater has been a proving ground for American playwrights – with their yearly one-act play marathon acting as a cheerleader for the 15-minute form. The mood is one of celebration, but audiences can also use it as a diagnostic tool. Boiled down into these relatively bald events, playwrights’ strengths and weaknesses stand out like Telly Savalas’s eyebrows. In Program B, what might pass for foibles in full-length productions emerge as crucial, sometimes fatal, flaws. Two quirky playwrights accidentally unmask their eccentricities and a titan of the American theater shows that he can make a really big thud when he falls. Luckily, Will Eno, whose reputation as an experimentalist precedes him, also exposes himself – this time, as a master storyteller.

Program B starts out with a whimper. Julia Cho, whose “BFE” masked mealy sentimentality with self-conscious oddball details, demonstrates how much more unbearable those characteristics are when concentrated. Despite delicate performances by Alison Bartlett, William Jackson Harper, and Diana Ruppe, “The 100 Most Beautiful Names of Todd” drips more saccharine sweetness than a shelf of Hallmark cards.

When Todd dies, his widow Louise (Ms. Bartlett) begins to make a list of his hundred best qualities, like “boiler of the perfect egg.”By the time her daughter Laura (Ms. Ruppe) actually says the line,”I called him … Dad,” the play is in sugar overload. Pity Mr. Harper in particular, who has to bluntly state the piece’s conceit (“It’s as though some all-knowing author put the perfect words in my mouth”). It is to the actor’s great credit that he manages to emerge relatively unsticky from this syrupy mess.

David Mamet’s piece could be summed up with a simple “it was bad.” But since we already know Mr. Mamet has an elephant-sized talent, it’s a bit impolite to ignore it when it sits in the living room. Barely eking out eight minutes of material (a subjective, non-stopwatched estimate), Mr. Mamet uses the same logic as the pricier restaurants: the better you expect the quality to be, the more insultingly thin they can slice the portions.

“Bone China,” directed by Curt Dempster (there is a cross to a phone and a blackout), imagines a psychiatrist’s session. A man (Victor Slezak) complains to his shrink (Marcia Jean Kurtz) that he’s sick of his daughter’s anorexia; he feels so fed up with her self-destruction that he no longer wants to stand in her suicidal way. His doctor, after clipping out a couple of Mametian sentence fragments, takes a call from a patient who is also bent on death. Then, the two of them sit sideby-side, their compassionate faculties completely shriveled. And … Finis! To paraphrase Mr. Mamet’s final line, “I think when we die (or enter play festivals), there’s an accounting. And some people (We’re looking at you, Dave!) just don’t care.”

“On the Sporadic,” by James Ryan, starts out with several of the boxes on the playwriting checklist x-ed off. Memorable characters? He’s got a diabetic, one-sixteenth Native American, vision-seeking drunkard (Jordan Gelber) making a false passport for a mysterious, uptight wreck (Ean Sheehy). So, check. An exciting scenario? The aforementioned drunkard rows the wreck into Canada, while both of them drift worryingly close to dementia. Double check. But that last important box, the one labeled “plot,” goes unfilled.

Mr. Ryan gets his characters into an incredible situation, and then absolutely refuses to get them through it.And as seasoned and impressive as both the sloppy Mr. Gelber and the tightly wound Mr. Sheehy are, they cannot tell an evaporating story. Eventually Mr. Ryan winds up introducing a telepathic witch (Greta Muller), simply to move things along. And when your show clocks in below 30 minutes, no one should have to rely on a telepathic witch.

But while the first three offerings of Series B disappoint in varying degrees, it remains a “can’t miss” evening.This is thanks to the incomparably deft “Intermission,” written by Mr. Eno, and sensitively directed by Michael Sexton. Their casting coups (the spectacular Jayne Houdyshell and Brian Murray) serve a piece that should be required viewing in the theater – especially for its critics.

Two couples (Mr. Murray and Ms. Houdyshell, Autumn Dornfeld and J.J. Kandel) chat with one another at a play’s intermission. From what we have heard, it sounds dreadful, which the cocky Jack (Mr.Kandel) points out. But his quibbles give way before Mr. Murray’s torrent of memory and invective. He doesn’t want to hear stylistic complaints, he wants the boy to recognize the play’s attempts at truth. And while Mr. Murray’s curmudgeon sneers at audiences’ yen for weeping at shows, Mr. Eno then makes us – practically by brute force – cry for him. Mr. Eno’s triumph is both canny and deeply touching, a vital look into a theater that actually reminds us what it’s for.

Until June 19 (549 W. 52nd Street, between Tenth and Eleventh Avenues, 212-352-3101).