The Washington Opera is back in the Terrace Theater -- where it emphatically belongs -- as vivid, intimate and finely polished as ever. Gaetano Donizetti's "The Daughter of the Regiment," which opened Saturday night, is an evening of thoroughly enjoyable, light-as-a-feather entertainment. Sung and spoken in English, it will remind audiences of Gilbert and Sullivan -- with more ambitious music.

There are problems in this season: There are only two Terrace productions rather than the four that have become traditional, and there is a serious ticket shortage.

And both Terrace productions (the other is Offenbach's "Christopher Columbus," which will open Saturday night) are pieces of pure froth -- with nothing as substantial as Britten's "The Turn of the Screw," Stravinsky's "The Rake's Progress" or Menotti's "The Medium," which have had such high impact in past Terrace seasons. Maybe the absence of serious themes explains the high ticket sales.

"The Daughter of the Regiment" is not exactly a bad opera but an unusually challenging one -- almost content-free and therefore highly dependent on style and technique. Fortunately, this production is a virtuoso performance from beginning to end, from the featured stars to the small-print supernumeraries who contribute tiny details with strong cumulative effect.

When the final curtain falls and then rises, it shows the entire cast, chorus and extras frozen to receive the first burst of applause. This is exactly right; the production is above all an ensemble triumph, and the whole group deserves recognition before soprano Erie Mills, mezzo Joyce Castle, tenor John Fowler and bass Franc,ois Loup take their well-earned solo bows.

The singing and acting are impressive, but what this production offers most spectacularly is style -- so much style that one is sometimes deluded momentarily into thinking that "The Daughter of the Regiment" may be about something.

The story (as usual in bel canto comedies) is one of true love thwarted and finally fulfilled. Marie, a foundling, has been raised from infancy by the 21st Grenadiers Regiment of Napoleon's Grande Arme'e, which she recognizes collectively as her "father." She has promised she will marry no one who is not a member of the regiment -- and then falls in love with a Tyrolean peasant, Tonio, who saved her life while she was gathering flowers on a mountain. Can she marry him? Father unanimously and emphatically says no; in fact, Tonio is arrested as a spy and is about to be executed when Marie tearfully throws herself in front of the firing squad.

The regiment, impressed, relents. Very quickly, Tonio joins the regiment and there are no further obstacles. But wait! Suddenly, it turns out that Marie is the long-lost niece (or could she be the illegitimate daughter?) of a local potentate, the Marquise of Berkenfeld, who takes her away from the regiment and off to her cha teau, where she will be transformed into a proper young lady and espoused to the insufferably effete Duke of Krackenthorp. The Grenadiers come to the rescue, of course, at the last minute.

A story like this may have been a box office smash in 1840, when "La Fille du Re'giment" was the hottest show in Paris, but what can you do with it in America in 1985? The Washington Opera uses it as a vehicle for spectacular singing (supplied bountifully by Mills and Fowler); comic acting, with which Castle and Loup come close to stealing the show; and inspired stage direction -- evidently a specialty of Leon Major. Scrubbed to a high polish, the production positively gleams -- and if most of the gleam is applied from outside, that does not diminish its brightness.

A few opening-night problems were purely peripheral. A grenadier's rifle barrel got bent in the middle of a flashy bit of close-order drill, and an apparently solid pillaring with some of his work. bit too vigorously. But the singing and acting offered little basis for complaint.

Mills handles her demanding music superbly and she is a first-class comedian who projects a warmly appealing personality across the footlights. Fowler is a rich-toned, agile tenor well versed in the bel canto style.

Any show that includes Loup automatically acquires a special comic polish and vitality. In this one, he plays opposite a singer and actress, Castle as the Marquise, who is fully his peer -- and I know no higher commendation. Their big scene together in Act 2 is a masterpiece of comic nuance, with occasional touches of slapstick -- as when Loup sits down to play a harp and the strings keep breaking. Castle plays a harpsichord -- really plays one on stage, with a fluent agogic accent. She parodies hilariously the mannerisms of a pretentious keyboard player: the long, meditative pause with head bowed before plunging into the keyboard; the precise arching of the wrist, the little twiddlings of the fingers, the rapt expressions, the annoyance with a singer who is not following her lead properly.

It is wonderful. So are two speaking actors, Jonathan Green and Muriel Smallwood, who create vivid caricatures, and Vladimir Ezkarkhov in the small singing role of a corporal. So are a multitude of details -- like the little doll Marie has kept from infancy, dressed in a grenadier uniform, a tiny touch that blends comedy, pathos and revelation of character. There is also a fine trumpet obbligato (played on an archaic instrument on stage), in which the trumpet and the prima donna match acrobatic melodies in what might almost be a parody of the flute-and-soprano episode from the mad scene of Donizetti's own "Lucia di Lammermoor." And much, much more.

Joseph Rescigno conducts the fine little orchestra stylishly and generates a great, thumping, chauvinistic sound when the music becomes military -- as it often does. But the music is not what you remember most from this production. what you remember is a "Daughter of the Regiment" funnier than one has any right to expect.