Due to an intriguing coincidence of the summer's release schedule, this weekend brings the simultaneous appearance of "The Secret of NIMH" and "TRON"--an event that led Film Comment magazine to run an article titled, "Will the Real Disney Please Stand Up?"

"NIMH" might be regarded as a Disney movie that got away. It is a "classically" animated fairy tale about the efforts of a widowed mouse called Mrs. Brisby to save one of her offspring, a critically ill tyke called Timothy. It is the first feature completed by a group of animators, led by director Don Bluth, who left prominent positions at the Disney studio three years ago to set up an independent production company.

"TRON" is a lavish, innovative gamble bankrolled by the Disney organization itself. Also a first feature, the brainstorm of a young, non-Disney animator named Steven Lisberger, it is a science-fiction escape thriller designed to showcase action sequences set inside the imaginary electronic universe of a gigantic, despotic computer system. This abstract environment, a stylized wilderness of grids, circuits and corridors inhabited by embodied "programs," the alter egos of the human programmers or "users" who created them, is visualized through an extensive, systematic use of computer-generated imagery, known as CGI in the trade.

The highly publicized defection of Bluth and his colleagues from the Disney stable of animators provoked a good deal of resentment back at the Disney plant and coincided with a decline in the company's filmmaking fortunes. "TRON" promised to give the floundering, demoralized studio, long confident of its public and its leadership in the art and technology of animation, a new lease on life. For the first time the techniques of computer animation, a stylish feature of many television commercials and logos, were being utilized in a big way on a feature-length film.

In addition, the premise of Lisberger's adventure fantasy anticipated a lucrative new form of amusement, computer games. One of the brainy young heroes of "TRON" is a brash programming whiz, Kevin Flynn, played by Jeff Bridges, who ends up a combatant in do-or-die enactments of the games he himself has programmed.

Both "NIMH" and "TRON" require an indulgent public, willing to take obvious but fitful signs of talent as a down payment on fully realized achievements. You certainly want to see what else they're capable of, because an imposing amount of graphic skill and sophistication is on display in these movies.

Bluth and his principal collaborators, Gary Goldman and John Pomeroy, a trio of close friends who were later joined by another dozen or so Disney defectors, have left the mood of "NIMH" peculiarly overbalanced on the dark, ominous side. Ironically, they were trailed out of the studio by a bad-mouth rap that accused them of being lightweights as well as ingrates. Somber to a fault, "NIMH" makes a potentially self-defeating mockery of the accusation.

There is a character designed to function as reliable comic relief, a stumblebum young crow named Jeremy, whose voice is dubbed by Dom DeLuise. Jeremy enters entangled in a strand of twine from which the admirable Mrs. Brisby frees him, the necessary prelude to a mutually beneficial friendship. Unfortunately, Jeremy keeps getting tangled up for the duration of the movie in repetitive ways that somehow fail to generate happy hilarity. He's entrusted with a virtual monopoly on the comic business, but he proves a defective comic performer.

The scenario, adapted from a superior, Newberry Award-winning children's novel, "Mrs. Frisby and the Rats of NIMH," by Robert C. O'Brien, suggests that Bluth and his colleagues possess a fairly astute storytelling sense. They've changed the heroine's name capriciously to Mrs. Brisby, but they've managed to repair a curious structural imbalance in the book, where the crisis surrounding Tiny Timothy was diminished by the emergence of a more compelling story, the epic saga of the rats of NIMH and their creation of a new civilization.

Mrs. Brisby and her four children occupy winter quarters inside a cinder block partially buried in a farm field. An early thaw threatens to bring out the farmer and his plow within a matter of days. The plowing is bound to churn up the Brisby habitat, where the ailing Timothy ought to rest undisturbed. Heeding the counsel of two sagacious woodland creatures--a venerable white mouse called Mr. Ages and a majestically terrifying owl, spoken with splendid vocal authority by John Carradine--Mrs. Brisby seeks out an advanced colony of rats who have powers far beyond the capabilities of other small mammals.

Their secret is that they evolved from a group of mutant experimental lab rats who outsmarted their keepers and escaped from captivity in the National Institute of Mental Health after being injected with chemicals and subjected to forms of training that elevated their intelligence. Indeed, the rats of NIMH learned how to read and reason as a result of their experience.

The desire of the Bluth group to sustain a richly illustrative tradition of fairy tale animation is also abundantly evident. Indeed, there's a tendency to overemphasize the pictorial versatility they want to recapture. The early sequences go out of their way to incorporate expressive shadows and reflections, and the decors have more detail and color range than the content really needs.

Despite a gripping plot and impressive illustrative style, "NIMH" isn't much fun. Often exceptionally skillful and sometimes exceptionally stirring, it nevertheless leaves something crucial to be desired. The unrelenting grimness was no doubt unintentional. Bluth must have imagined that Jeremy gave "NIMH" the comic relief and buoyancy it desperately lacks.

"TRON," on the other hand, seems to suffer from a wandering, diffuse continuity from the outset, when Lisberger tries to establish the parallel worlds of ordinary reality and the computer system much too abruptly. Flynn is attempting to confirm his well-founded suspicion that a rival programmer, Dillinger, played by David Warner, stole his video games and exploited their success as a vehicle of self-promotion. As a matter of fact, Dillinger's deviousness has gone so far that it threatens to consume him: The Master Control Program he invented as a vehicle for snooping and program-stealing has taken on a tyrannical life of its own. It's bossing around Dillinger and appropriating computer data from all over. When we first encounter MCP, in fact, it expresses an insatiable desire to swallow up the systems at the Pentagon and Kremlin and get on with the business of running the planet, since it's "900 to 1,200 times more proficient than any human system."

Flynn, who has left ENCOM, tries unsuccessfully to gain access to his lost data through a home terminal. His ex-girlfriend, Lora (Cindy Morgan), remains at the company, working with an elderly colleague on a laser that can transform organic objects into digital information and then reanimate them. Her new boyfriend, Alan (Bruce Boxleitner), is another software engineer who finds that he has lost access to one of his programs, a security program called TRON.

Flynn finds himself transported to a universe of ominous geometric proportions, but also discovers allies in this alternate universe: Tron and Yori, the alter egos of Alan and Lora. Together they form an intrepid trio and set out to overthrow the MCP and restore freedom and access to the ENCOM system.

The heroes bear obvious resemblances to the three principals of "Star Wars." If "TRON" were an effectively constructed movie, these resemblances might have seemed both amusing and serviceable. Flynn, Alan and Lora are unusually brainy characters for the movies, and they appear very promising when first introduced in the real world. Unfortunately, the promise doesn't extend beyond the introductory stage, and once we're inside the computer system, there's little differentiation between them, in terms of personalities or functions. We don't even get a humorous sense of the difference between Flynn's transformed entity, who is not, despite his miniaturization, a program, and Tron and Yori, who are.

It couldn't have been easy, of course, to orchestrate the continuity of an adventure movie in which most of the action takes place in an essentially invisible setting, but it's Lisberger's failure to orchestrate this aspect of the show that ultimately causes the picture to sag. Fascinating as they are as discreet sequences, the computer-animated episodes don't build dramatically. They remain a miscellaneous form of abstract spectacle. The escape and the assault on the MCP ought to create some kind of ascending, intensifying melodramatic curve. Ironically, they have no effective shape at all in an environment specially programmed for geometrical sophistication.