Eighteen years ago, Thomas Harris was living proof that F. Scott Fitzgerald had it all wrong. In a bravura rebuke to the idea that there are no second acts in America, Mr. Harris had, with “The Silence of the Lambs” (1988), achieved the improbable: writing an even greater thriller than the great one that inspired it, his own “Red Dragon” (1981).

The earlier book had struck millions of readers as the most accomplished, and simply terrifying, suspense novel of its time. Among other things, “Red Dragon” introduced to a general readership the concept of “profiling,” a forensic science unknown outside of Quantico at that time, and the notion of a “serial killer,” a term coined by the FBI only a few years earlier and not yet in the popular lexicon. The novel also introduced the most deliciously malevolent fictional character in recent memory, Dr. Hannibal Lecter — though only in a supporting role in a fiendishly intricate story about a desperate man-hunter, the retired FBI agent Will Graham, and his savage prey, the eponymous Francis Dolarhyde, a twisted lover of William Blake’s phantasmagorical art and heartless butcher of helpless families.

Lecter returned seven years later, though still as a secondary character, in “The Silence of the Lambs.” The protagonist of that story was Clarice Starling, an FBI trainee in pursuit of “Buffalo Bill,” a slayer and flayer of women who fashioned grotesque uses for his victims’ flesh. In what amounted to an almost religious allegory, Lecter, a malign deus ex machina, battled with Starling’s supervisor, Jack Crawford, for possession of Clarice’s soul — and never before had a popular novel so clearly pictured the simultaneous attraction and repulsion of pure evil. A triumph of the imagination, “The Silence of the Lambs” transcended the thriller genre through Mr. Harris’s artistry in grounding fate in unfolding character rather than mechanical plot dynamics.

And then, after both the original novel and an Academy Award-winning film adaptation (1991) of “The Silence of the Lambs” had topped the charts, Mr. Harris tried for a third act in 1999. But how could he continue to best himself? In “Hannibal,” his answer was not to try, but rather to abandon the genre and nearly all of its formal mechanisms — suspense, procedural detail — in favor of a lurid Grand Guignol sketch that mixed improbable plot developments (Starling quitting the FBI and falling in love with Lecter) with absurdist detail (Lecter dining on the brain matter of a living but unaware victim).

It would be kind to say that “Hannibal” was a disappointment. Absent were the shrewd psychological insights (into Starling’s ambitions and insecurities, for example) and casually sprinkled but closely observed sociological details (about postgraduate students’ one-upmanship; about cars as indicators of social class) that gave his earlier books such a remarkably rich texture. In their place were rhetorical flights of fancy that displayed a tourist’s view of Italy and a researcher’s, rather than a novelist’s, understanding of abnormal psychology.

Most troubling of all, Lecter had taken over center stage, and readers had to view him virtually on his own, rather than through his duels with strong foils such as Crawford or Graham. Mr. Harris seemed to have forgotten that Lecter was a secondary character, not a leading man. The evil deus ex machina had become an inadvertent laughingstock. Imagine Professor Moriarty without Sherlock Holmes, morphing into Jerry Lewis without Dean Martin, in full-flop sweat during the bathos of yet another telethon.

After the letdown of “Hannibal,” it would give me enormous pleasure to declare that Mr. Harris had yet again defied the odds, and Fitzgerald, with his new book, “Hannibal Rising” (Delacorte, 323 pages, $27.95). But this fourth act in the Lecter saga is more like a series of notes for a novel than a fully realized work of the imagination.

Gone is the intricate plotting that struck readers of “Red Dragon” and “The Silence of the Lambs” as both utterly unpredictable in the devouring and satisfyingly inevitable after the feast. Gone, too, are the complex conflicts among characters struggling with their own mixed motives.

Instead, “Hannibal Rising” has an almost straight-line revenge plot. Lecter, a young teenager in war-ravaged Lithuania in the 1940s, sees his life destroyed by successive waves of Nazis, homegrown predators, and communists. His ancestral home is looted, trashed, and confiscated; his parents — local aristocrats — are brutally killed. And then his beloved little sister suffers a fate that can only be described as painfully obvious from page one, given what readers already know about Lecter’s gastronomic predilections in the other books.

To the surprise of no reader, but to the apparent astonishment of every other character in the book, Hannibal is determined to hunt down and exact retribution from the brutes who have crushed what remained of his innocence. Since we already know the character will survive this “prequel” and live through three more books as an adult, I don’t think I’m spoiling anything by noting that the outcome is never in doubt.

Worst of all, Mr. Harris has forgotten what his characters knew about Lecter in the earlier (i.e., later in fictional time) books — that he is not just evil but inhuman. Crawford calls him a monster, because, he says, no one has invented any other term that properly describes him.

At times in “Hannibal Rising,” the other characters also refer to the young Lecter as a monster. But what Mr. Harris shows belies this description. This polite young man could never plausibly develop into the brilliant and frightening sociopath whom Graham, Crawford, and Starling labored so hard and so long to remove from civilized society.

In “Hannibal Rising,” there is none of the horror and pity aroused by Mr. Harris’s account of Dolarhyde’s less epic but more appalling childhood in “Red Dragon.” There is only a perfunctory sketch of wartime suffering that pales in comparison to countless harrowing true-life accounts we are all familiar with. By picturing Hannibal’s plight during the war and by having him repeatedly struggle with the memory of his sister’s fate, Mr. Harris has reduced the malign deus ex machina of “Red Dragon” and “The Silence of the Lambs” to just another head case suffering post-traumatic shock disorder.