A fierce slice of Verdi is playing at the Metropolitan Opera. On Monday night, the company revived “Un Ballo in Maschera” (“A Masked Ball”), one of the most Verdian of operas. Are some Verdi operas more Verdian than others? I think so, but we don’t have time for that discussion now. As for the plot of “Un Ballo,” it involves royal intrigue, forbidden love, disguise, betrayal, misunderstanding … In short, this is an opera.

Monday night contained two performances, in effect: One mediocre to poor, the other quite good. The good performance started, really, with Act III – and though this is the final act, it was not too late.

James Conlon conducted, and we were cheated immediately, as the opening pizzicato playing was imprecise. Mr. Conlon proceeded in a stiff, mechanical manner – Dullsville. This reminded me of his performance of Mahler’s First Symphony with the New York Philharmonic earlier in the season, which was one of the most pedestrian Mahler performances imaginable. You may think that the “Titan” Symphony cannot be dull, but you didn’t hear this.

In the Verdi, phrase after phrase seemed placed, rather than natural. And we saw that forceful gesticulation does not equal musical excitement.

The opening chorus had no warmth, color, or lift – and coordination was lacking between pit and stage. As Act I continued, several tempos were sluggish, or stilted, and the orchestra was often too loud for singers: for the Oscar, for one. Act II began with a nice vigor and cohesiveness, but the intensity of Verdi was largely missing. The great soprano-tenor duet in Act II was badly shaped – stiff again – with its climax (“Si, t’amo!”) hardly climactic.

The tenor portraying the King was Marcello Giordani, who began sharp and unsure. (Sharp in pitch, I mean.) Also, he was quite stiff, in keeping with the theme of the evening (or rather, about half of it). He had trouble with quick, multi-syllabic phrases, and he had very little lower register – notes that are required in Act I, Scene 2. In the aforementioned duet, he was heroic, if not terribly musical – this is a tenor prone to bluffness.

Renato was Carlos Alvarez, a Spanish baritone who owns a gorgeous and striking instrument – sort of on the wet side. Some voices have a juiciness, an inexplicable phenomenon. Mr. Alvarez had some shaky moments early on, but also some impressive ones, as when he delivered a beautiful, free high G.

The soprano singing Oscar – this is a pants role – was Lyubov Petrova, whose sound, as always, was bright and forward, but also slightly strident. To her credit was good intonation, and fairly agile coloratura, or semi-coloratura. Theatrically, Ms. Petrova made a spunky and apt page.

Ulrica was Marianne Cornetti, “a real Verdi mezzo,” as we say. Her big aria could have been scarier – more vivid, more slashing – but it was adequate. It probably did not help her that she was toward the back of the stage, and that the orchestra frequently covered her.

And now to our soprano – our main soprano – Deborah Voigt. She was again Amelia, the role in which she made her Met debut, in 1991. This time, she had a terrible beginning, and far from her best night, overall. Absent was the famous Voigt creaminess, replaced by a hard edge. And in her first scene, she was badly, badly flat, especially on high notes. She was miles below a couple of Bs, and one C. In the duet, she got her C – got the pitch – but it was unattractive, to put it mildly. (Thus it was un-Voigtian.)

But wait: The opera pretty much woke up in Act III, beginning with Amelia’s aria, “Morro, ma prima in grazia.” It was a little slow – bordering on static, in spots – but it was poignant. And accurate. Plus, Ms. Voigt sang it on her knees, or prone.

Which brings us to the question of: the weight loss. As is news all over the world, Ms. Voigt has lost a huge amount of weight, after years of struggle. (Does the struggle ever end?) I remember the tenor Ben Heppner’s first Met performance after a similar weight loss: He positively reveled in his new physical possibilities, falling to his knees, for example, and springing right back up. He raced around the stage – in “Les Troyens” – whenever he could. Ms. Voigt, too, seemed to revel in new possibilities. And why not?

Also, note this: When Ms. Voigt sings well, critics and others may attribute that to the new body; when she sings badly, they may attribute that, too, to the transformation. One should be skeptical of such attributions. Ms. Voigt fluctuated before, too, and not just bodily.

But back to Verdi: The “Eri tu” of Mr. Alvarez was stirring, classically rendered, and Mr. Giordani gained some new powers – musical and theatrical powers. He was still sharp, but he was convincing: The King’s death was very well sung and acted. As for Mr. Conlon, he seemed a new man, pacing the third act superbly. We were hurtling toward that masked ball, “the dance of death.” When the three would-be assassins drew lots, Mr. Conlon provided a savage intensity. And the final chorus was rightly heavenly – despite a reversion to flatness by Ms. Voigt.

Hao Jiang Tian and Paul Plishka were consistently excellent as the two conspiring counts, and Brian Davis showed a smooth, confident baritone for Silvano. Notable in the orchestra was a gorgeous, swoony cello introducing Amelia’s aria, and a beautiful, Viennesy violin to dance to, at the ball. Piero Faggioni’s production is supreme, with Laurie Feldman’s stage direction smart and moving. And Verdi! He knew what he was doing: “Un Ballo in Maschera” is tight, ingenious, and smashing.

“Un Ballo in Maschera” will be performed again April 7, 11, 14, 18 & 22 at 8 p.m. and April 25 & 28 and May 3 at 7:30 p.m. (Lincoln Center, 212-362-6000).