LEONARDO DA VINCI'S ''Last Supper'' in the refectory of Santa Maria Delle Grazie, completed in 1498, counts among a handful of images that divide art history into Before and After. Yet its original glory soon fled. Painted in an experimental and, as it proved, highly perishable fresco technique, the work began to show marked deterioration by 1517. Another half century later, Giorgio Vasari, author of ''The Lives of the Most Eminent Italian Architects, Painters and Sculptors,'' considered ''The Last Supper'' no more than a ''macchia abbagliata,'' a phrase difficult to interpret, perhaps best rendered as a ''bewildering smudge.''
In the centuries that followed, matters got worse. Apart from the inherent instability of Leonardo's medium, ''The Last Supper'' suffered repeated overpainting, target practice by French soldiers in the 18th century and the bombs of the Allies in the 20th. Yet through copies and copies of copies, its influence persisted. Imagine: At the corner of Lexington Avenue and 106th Street, in Spanish Harlem, an urban muralist has splashed a funky take of the painting across a brick wall.
On view through June 17 at the Palazzo Reale here, the exhibition ''Genius and the Passions: Leonardo and the Last Supper'' examines what it calls the ''precedents, innovations and reflections'' of this singular creation. The exhibition is the latest of several recent blockbusters at this labyrinthine complex. And like such predecessors as ''The Soul and the Face,'' ''Lombardy in the 16th Century: From Leonardo to Caravaggio'' and ''Verdi: The Man, the Works, the Myth,'' this one pursues an ambitious intellectual agenda through sprawling displays of splendors.
The title bears a few words of explanation. ''By 'genius,' I mean the genius of the art of painting itself as it reveals itself through Leonardo,'' the show's curator, Pietro C. Marani, said at the gallery last month. At the same time, he admitted that in fact Leonardo's expression of the passions in ''The Last Supper'' is without precedent.
A professor of art history, a former museum director and a widely published author -- his scrupulous, lavishly illustrated ''Leonardo da Vinci: The Complete Paintings,'' published in English last year by Harry N. Abrams (384 pages, $85), summarizes the state of knowledge in its field -- Mr. Marani combines the learning of a scholar with the flair of a showman. In a surprise prelude of great conceptual elegance, an opening gallery presents an assortment of historical photographs of ''The Last Supper.'' Supposedly documentary in intent, many of them have been extensively retouched. These pious frauds announce what will emerge as Mr. Marani's general theme: that throughout its history, the fresco has reached new generations through intermediaries intent on undoing the ravages of time.
At this point, the plan of the exhibition turns chronological, going back to the early 12th century. Let the record show that Leonardo was not the first to seat his full cast of 13 on the far side of a long table, but other arrangements abound: the earliest item in the catalog, a miniature on parchment, has a half-moon table, its straight edge toward the viewer. Judas stands isolated in the foreground, eating a morsel actually placed in his mouth by the hand of Jesus, as the 11 others look on. There are round tables, neatly symbolical, with Jesus at 12 o'clock, John (as usual) asleep beside him, and Judas (his back to the viewer) at 6. Elsewhere, at a rectangular table, Jesus takes the head, Judas at his right hand as the others spread out evenly around the three other sides. Such variations continue well beyond Leonardo (Titian places the table on a diagonal), but from 1498 onward, Leonardo's scheme is the fixed point of reference.
The early depictions of ''The Last Supper'' in miniatures, tapestries, drawings and frescoes (some distinctly Byzantine in flavor) have one crucial quality in common: all of them convey the pathos of the scene through a sort of diffuse solemnity. None remotely anticipates Leonardo, who through genius of portraiture and gesture dramatizes a single sharp, specific moment.
''He that dippeth his hand with me in the dish, that same shall betray me.'' The fatal words still hang in the air. The face of Jesus reflects drained resignation, but around him rages a symphony of individual reactions: puzzlement, suspicion, brooding, anxiety, terror, denial, the search for allies and (most moving of all) the stark, lonely revulsion of James Major, caught in full exclamation.
The moment when the world first laid eyes on this tremendous image is beyond recall. Yet the ingenious Mr. Marani brings us amazingly close. A group of preparatory drawings gleaned from the richest collections in the world takes us deep into Leonardo's imagination and technique. Any museum would be proud to present them as a show unto themselves, but here they occupy a dark rotunda from which we exit to face Giampietrino's full-size copy of the fresco, from about 1515. Though Giampietrino captures none of the aura we associate with Leonardo, he gives a scrupulous (if sometimes coarse) inventory of what Leonardo painted: the view through the windows, the architecture of the room, the setting of the table. The colors, in garish oils, can never have matched the more subdued palette of fresco, yet they, too, help wake us up to the effect Leonardo made on his contemporaries. Talk about the shock of the new.
From here, Mr. Marani conducts a stroll through five centuries of Last Suppers, past Tullio Lombardo's in marble, Andrea da Milano and Alberto da Lodi's in polychrome and gilded poplar wood, Anthony van Dyck's baroque homage to Leonardo, Blake's ''Gothic'' reenactment, on to parodies in stills from films of Luis Buñuel and Pier Paolo Pasolini, a DayGlo silk-screen treatment from Andy Warhol, and finally Daniel Spoerri's folk-art takes involving a copy of Leonardo on a velvet rug and a dollhouse cabinet's worth of miniature tableware. Never mind that we sometimes derail for an irrelevant Giorgione or Caravaggio or Degas: they are worth stopping for.
Hogarth, too, puts in a gratuitous appearance. Are there really formal allusions to ''The Last Supper'' in ''Cockpit'' and ''Columbus Breaking the Egg''? Doubtful. Still, in a certain frame of mind you start to see them everywhere -- at the Pizzeria Vesuvio in Vicenza, for instance, where I stopped the Sunday after seeing the show. In walked an extended family and took their places like born disciples of Leonardo.
But rather than give in to this little idée fixe, I suggest viewers of ''Genius and the Passions'' proceed straight from the Palazzo Reale to Santa Maria delle Grazie to see Leonardo's magnum opus firsthand. Access is strictly controlled; two years after the completion of a 21-year restoration project, reservations are a must.
No one who looked in to see ''The Last Supper'' in the course of restoration (or remembers it as obliterated from before) will imagine what the conservation has accomplished. The work has in one sense been very humble. The fresco remains a ruin. But with layer after layer of obfuscation gone, what remains of the original speaks with the force of revelation. We see through a glass darkly, but we see.
Pinin Brambilla Barcilon, chief conservator on the project, ranks with Mr. Marani as the world's foremost authority on the fresco and is his co-author of ''Leonardo: The Last Supper,'' published in English this year by the University of Chicago Press (440 pages, $95). The historical, aesthetic and technical analysis is exhaustive, and over 200 pages of large-format post-conservation photographs bring a masterpiece before us square inch by square inch. All this for a bewildering smudge? After Ms. Barcilon's labors (and Mr. Marani' show), that tag -- old as the Renaissance! -- no longer applies.