Riccardo Zandonai's "Francesca da Rimini" separates romantic slobs from other kinds of slobs. In fact, the difference between these two kinds of slobs is a basic theme of this fragile opera expanded from a few lines in Dante's "Divine Comedy." It can be seen tonight on PBS in the Metropolitan Opera's lavish production (8 p.m., Channel 26 and Maryland Public TV; simulcasts on WBJC-FM, WETA-FM and WGMS AM and FM).

The slob contrast is most sharply pointed in the first scene of Act 4, where most of the opera's dramatic action takes place. The heroine, Francesca (Renata Scotto), is being chased around her 13th-century castle by her lustful brother-in-law, the one-eyed Malatestino (played with an almost terrifying power by tenor William Lewis). Malatestino offers to poison Francesca's unloved husband, Gianciotto, who is conveniently absent. Francesca shouts, "Do not touch me, you madman!"

She is disturbed by the moans of a prisoner being tortured in a nearby dungeon, and she makes the mistake of complaining about it to Malatestino. "I shall see that you have a peaceful night," he pledges. Then he grabs a torch and a battle-ax and sets off for the dungeon. A few minutes later, he emerges with the prisoner's head in a burlap sack -- but by then Francesca has run away, screaming.

Can true love survive in an environment as crude as the Malatesta family castle? That is the question raised by this opera, and the answer is: not for long. The story (based on a historic incident) is one of adultery, beginning in poetic ecstasy and ending in abrupt, violent death.

Francesca has two brothers-in-law; the second, Paolo, is also a tenor (Placido Domingo, at the top of his form), but in every other way he contrasts with his brothers, and Francesca cannot help falling in love with him. His job is fighting -- the Malatestas were condottieri -- free-lance military leaders in the intense, complicated feudal wars of medieval Italy. But like Francesca, he has the soul of a poet. Together, reading a love scene in the old romance of Lancelot and Guinevere, they are overcome with passion. When the rejected Malatestino reveals their adultery to Gianciotto (the always reliable Cornell MacNeill), the story rushes to a quick, bloody conclusion.

"Francesca da Rimini" alternates between scenes of love and of violence. Both are well handled in the Met production, but the love scenes between Domingo and Scotto that conclude Acts 1 and 3 are the moments that linger longest and most vividly in memory. The singing is good (even with one or two small wobbles in Scotto's voice), but some of the best moments are wordless and they are superbly handled by the great Metropolitan Opera Orchestra under the baton of James Levine.

Romance reaches its apogee in a long, ecstatic, wordless scene at the end of Act 1, when Francesca meets Paolo for the first time, mistakes him for her unknown fiance', picks a rose and silently presents it to him while an eloquent solo cello pours out feelings too deep for words. For romantic slobs, it is unforgettable.

Extraordinary care was taken with the visual preparation of this production, and it is faithfully reflected in the camera work that will be shown tonight. Again and again, the picture from the Met's stage has the same qualities as the Italian paintings that are shown during an intermission feature on the historic Paolo and Francesca.