"Cradles for cats/ Are string and air./ If you let go/ There's nothing there." Thus runs a duet sung (while playing with a cat's cradle) by Mrs. Grose, the simple, warm-hearted housekeeper, and little Flora, a pre-school-aged Victorian child, as Benjamin Britten's opera "The Turn of the Screw" twists toward its shattering, ambiguous conclusion.

The little nursery song could almost be a description of the opera itself: a tight, intricate structure that is held together, really, only through tensions set up and maintained in the minds of the audience. If you let go, there's nothing there.

Dramatically, it is a fragile structure of hints, innuendos, frissons: things half-glimpsed and dimly understood. Being by Britten, the music is tougher and more substantial; the opera is constructed, in fact, around one of music's tightest, most flexible and most intricate forms: a theme and variations. In flavor, the music is nervous, punctuated by jittery bursts of percussion, and often high-pitched. The lowest voice is the tenor's, and he uses it most often in a very high register.

"The Turn of the Screw" is undoubtedly a masterpiece, and the Washington Opera opened a very strong production of it last night in the Terrace Theater. But it is hardly a masterpiece to everyone's taste -- hardly the kind of entertainment to which one would automatically turn after enjoying "Tosca" or "Cenerentola." It is this year's gamble by the Washington Opera: contemporary music sung in English, when everyone knows that operas are things written a century ago in a foreign language -- things with lots of tunes. In this, "Turn of the Screw" corresponds to "The Rake's Progress," which was last year's gamble and a smashing success. But it is much more of a gamble than "Rake," which has not only tunes but a strong circus element: a bearded lady, an auction scene and a brothel scene, lots of crowd action and a jovial devil -- jovial at least until near the final curtain. "Turn of the Screw" is a stark experience, although it does indulge in canny patterns of tension and relaxation and reaches smashing climaxes at the end of both acts. It is dead serious, lacking the "Rake's" touch of self-mockery, which serves as protection against the mockery of others. It requires of the audience a very high level of involvement; if you let go, there's nothing there.

Not much actually happens in the opera's 18 short scenes: a rather troubled (perhaps neurotic and sexually obsessed) young lady comes to a British country house, where she will be the head of household and governess to two lonely little orphan children, Miles and Flora. She hears rather vague but ominous rumors about Peter Quint, a former manservant, and Miss Jessel, a former governess, who are now dead. She thinks she sees them coming back to take the children, trying to woo them into another reality. She writes a letter to the aloof, distant uncle, "a young man, bold, offhand and gay, the children's only relative." The letter disappears; Flora is taken off to the uncle by Mrs. Grose. A final confrontation with Miles over the stolen letter leads him to repudiate Quint's influence, shouting "Peter Quint, you devil" -- after which he dies in the arms of the governess.

Nothing is really settled at the end. The opera is a struggle between two worlds (those of the ghosts and of the governess) for control of a third: that of the children. But who is the winner? All is ambiguity and uncertainty. In this, the opera is faithful to the book, though it simplifies greatly and changes some details. It may be (in the spirit if not the letter) the most faithful operatic rendering of a work of literature since Berg's "Wozzeck," in which the original text is simply set to music.

The casting for this production is very strong, though there were some opening-night problems that should be resolved as the six singers settle into their roles -- largely problems of clarity in small gestures and the projection of words. Perhaps 20 percent of the libretto could not be understood on opening night -- an unacceptable ratio with a lucid English text and acoustics like those of the Terrace. Two of the singers, Barbara Hocher (Mrs. Grose) and Elizabeth Knighton (Miss Jessel) had this problem at the beginning but conquered it by Act II and ended with strong performances. The two children were played with remarkable skill by Ashley Leadbetter and particularly by Laurence Pittenger -- a young man with a fine voice and striking stage presence -- but in terms of clarity their diction was on-again, off-again, possibly complicated by the acoustic effects of various positions on the stage.

The most complete success of the evening was tenor Dennis Bailey, from the beginning (when he appeared in the middle of the audience to sing the prologue) to the end. His bright, flexible voice mastered every nuance of the sinuous music composed originally for Peter Pears (a very hard act to follow), and every syllable was charged with meaning. His acting embodied not so much the sinister aspects of evil but its attractiveness -- a much more chilling note. Comparing this performance to his vocally fine but theatrically problematic role in "Carmen," one suspects that he finds Quint more interesting than Jose' -- perhaps a sign of good taste.

Susan Peterson sometimes seemed nervous and tentative as the governess, but it is a nervous and tentative kind of role. Vocally, she was at home in her highly demanding music, bringing her voice more than once to the verge of hysteria without losing control. Theatrically, she was nearly ideal -- though her appearance, striking distraught poses in Victorian costume amid Jean-Claude Maret's ingenious, stylized scenery, sometimes had an effect near that of a Gorey cartoon.

John Mauceri conducted a taut, beautifully paced musical interpretation with a few orchestral details that will certainly be polished for repeat performances.