Which diva will sing for simulcast?

Whatever happens diva-wise on Saturday, the Metropolitan Opera's simulcast of Gluck's

Iphigenie en Tauride

explores territory seldom frequented by this august company. And how much anybody will enjoy being there depends, to some extent, on what happens diva-wise.

Mezzo-soprano Susan Graham has canceled several times due to illness, most recently Monday, on the afternoon of the show. She alone would be reason to catch Saturday's simulcast, given her voice, charisma, and specialty in all things French, but especially since this opera is one of the few that give the leading role to her voice type. Among her costars is 70-year-old tenor Placido Domingo, who sings the baritone role of Oreste transposed up a bit, allowing him to still sound like himself and giving him good dramatic fodder (his character hallucinates) without having to carry the entire opera. But should Graham cancel, her designated replacement, Elizabeth Bishop, is highly regarded; once she settled into the role on Monday, her Iphigenie was thoroughly satisfying vocally and dramatically (even if the costumes, conceived for the tall-ish Graham, didn't enhance her stage presence).

Where divas make a difference, though, is in how the opera itself comes across. Though works like Carmen are relatively immune to casting decisions of varying quality, Gluck is not so sturdy. Premiered in 1779, Iphigenie is the summit of his creative life, the work of a composer who envisioned a kind of proto-Wagnerian theater unfolding in a continuous stream of music, not driven by vocal display but words and characters (this time based in the Greek tragedy of Euripides) experiencing profound inner transformation. But he wasn't always up to realizing these intentions convincingly. Compared to Wagner (not fair but unavoidable), the opera seems like one of those quaint, Jules Verne flying machines trying to do the work of a jumbo jet. Shrewdly, conductor Patrick Summers goes for low-vibrato textures to achieve maximum detail in what can seem like an opaque, plain orchestration.

Iphigenie has some external action - the long-exiled title character discovers that her long-lost brother Oreste is now a fellow prisoner in primitive Tauride - but the important plot points are her internal dilemma expressed in grave, contemplative soliloquies. She could rescue Oreste, but he'd rather die for his friend Pylade. (Since the opera was written for Paris, action-interrupting dance interludes were obligatory. But flying machines will surprise you.) Iphigenie's pre-intermission aria, "Oh malheureuse Iphigenie," isn't just exquisite but creates a complicated emotion with great musical simplicity. Unexpectedly written in a major key, the aria has Iphigenie finding purification in her suffering.

Director Stephen Wadsworth has built his career around successfully reanimating mythology, and to that end shows Tauride as a primitive, torch-lighted hideout for a radical religious community. The music's Turkish influence accounts for the presence of whirling dervishes, and more eccentric forms of movement. Magic isn't unusual in this world: In the first scene, Iphigenie is saved from a sacrificial death when the goddess Diana descends on stage wires and scoops her up. Less stage-savvy is the way Oreste's hallucinations are externalized. More important than any of this, though, is that the singers all know what they're about, and project that passionately.

Lucky for Domingo that he's long been willing to go to the mat for any role, because entering the less-heroic baritone zone means having to do more theatrical dirty work. He spends much of the time chained to a sacrificial altar in various states of agony, though always singing with elegance, clarity, and a willingness to change his timbre to suit the French language.

Though an excellent tenor, Paul Groves as Pylade has a little trouble being noticed next to a high-octane personality like Domingo. But in simulcasts, the lower-key singers often register most convincingly on the big screen.