A movie that succeeds in visualizing an imposing, frightening new monster is nothing to be sneezed at. The principal claim to fame of "Dragonslayer," now showing at area theaters, is that it launches a monster fit to join the legendary company of King Kong, Godzilla, the shark in "jaws" and the intruder in "Alien." It also boasts the classiest handle in monster annals: Vermithrax Pejorative.

Verm, for short, is a mammoth, winged, fire-breathing dragon, the aging and terrifying curse of Urland, a fanciful region of the British Isles in the 6th century A.D. Writer-director Matthew Robbins and writer-producer Hal Barwood have attempted to synthesize a spectacular new adventure-horror fantasy out of their obvious nostalgic affection for a number of far-flung sources: the St. George legend, the "Sorcerer's Apprentice" and "Night on Bald Mountain" sequences from Disney's "Fantasia," Shirley Jackson's short story "The Lottery," Fritz Lang's "siegfried," Cecil B. DeMille's "the Ten Commandments," George Lucas' "Star Wars" and Steven Spielberg's "close Encounters of the Third Kind."

There are direct stylistic influences from Lucas and Spielberg, who are friends of Robbins and Barwood. In addition, several key members of the crew -- cinematographer Derek Vanlint and special effects supervisor Brian Johnson, for example -- came to "Dragonslayer" directly from "Alien" or "The Empire Strikes Back" or both (in Johnson's case). The Disney studio was also involved.

An adventure fantasy this ambitious runs the risk of leaving a small margin for error. Either it delivers sensations sufficient to knock patrons out of their socks or it falls perilously short, betraying expectations of unique and transporting movie wizardry.

Robbins and Barwood have a magnificent monster, but they're as unreliable as their protagonist, a daring young sorcerer's apprentice played by newcomer Peter MacNicol, when it comes to manipulating the storytelling, performing and atmospheric resources at their command. Beautifully lit, impressively designed, cleverly plotted and nightmarishly exciting when Vermithrax is on a fiery rampage, "Dragonslayer" nevertheless leaves several disillusioning miscalculations and shortcomings in its wake.

The screenplay probably owes more to "Star Wars" than any other single inspiration. MacNicol's Galen suggests a medieval Luke Skywalker. His mentor, the aged sorcerer Ulrich, impersonated by the great (and underutilized) Ralph Richardson, is obviously a variation on Ben Kenobi.

Valerian, the heroine of "dragonslayer," shares certain obvious attributes of Princess Leia. She's a fearless, abrasive type, and it's her request for Ulrich's assistance that sets the adventue in motion. However, there are a couple of entertaining switches with Valerian, played by another newcomer, Caitlin Clarke. She's a poor man's daughter, and sulkily class-conscious enough to qualify as a prehistoric New Leftist. She's also disguised as a boy in the early stages of the story. The king of Urland has attempted to control the wrath of Vermithrax by offering it a virgin sacrifice every so often. Valerian's father, a village blacksmith, has protected his daughter from the lottery that determines the unfortunate virgin by concealing her sex since birth.

As a movie concept, "dragonslayer" seems to have so much going for it that it could scarcely miss. Yet it does miss in crucial respects. Robbins and Barwood play trickier games with the plot than they need to, defusing an apparently straightforward, dynamite dilemma -- how can two intrepid young people destroy a fantastic beast? -- with needless subsidiary complications and anticlimactic hitches. MacNicol and Clarke also prove an unprepossessing match. When the exposition falters or the action misfires, "dragonslayer" has little to fall back on. The surest personality resources -- Richardson as Ulrich and Sydney Bromley as his wonderful old servant Hodge -- are removed from the scene much too early. The kids can't really compensate for their absence.

Still, there's always Vermithrax, and when Galen does battle with the beast in a cavernous grotto, many a primitive, childish vision of hellfire and satanic torment may come vividly alive again. "dragonslayer" is certainly at its most effective and imaginative when Vermithrax comes closest to scaring you devout. While the "PG" seems justified, given the fantastic context and the relatively brief bits of gruesomeness, it's usually best to err on the side of caution when speculating about the effects of frightening images on the littlest kids.