Valerie catches on to what is happening rather late in this blood-soaked mystery. "It's like a horror movie," she says, clutching the stalwart, 16-year-old arm of Scott Gardiner, who is making it happen -- or at least part of it. They are in a dark, drafty corridor of Ballycastle, an old Irish structure transported stone by stone to the banks of the Hudson in Upstate New York half a century ago and rebuilt to satisfy the whim of an eccentric millionaire.

The castle has a poor reputation, historically and in its current neighborhood -- and there may be solid reasons. It is riddled with secret passages, guarded by a half-crazy old man who was brought over from the Old Country along with the segments of the castle. Something that seems not quite human but more than animal (and thoroughly demented, in any case) roams the grounds by night, scaring people and wreaking havoc. On a hilltop not far away is a little cemetery full of the graves of servant girls who came over from Ireland in the 1930s and died by the time they were 20. A foundation has taken over the estate and is trying to develop it as a tourist attraction, but it gets little help from the residents of the nearby town, Flat Rock.

So why are the kids of the local high school holding a dance in the castle? Why have they made it a costume ball, with the costumes based on a game called "Hobgoblin" that most of the kids in the school consider either too dumb or too complicated to be interesting? These are only a few of the mysteries in the latest novel by John Coyne (author of "The Legacy," "The Searing" and "The Piercing"), but they are the wrong kinds of mysteries; they are loose ends and failures of character motivation that fledgling novelists are supposed to have had drilled out of them in their first creative writing course.

Valerie is right; "Hobgoblin" is like a horror movie -- specifically the genre of horror movie that focuses on the offing of teen-agers in colorful ways. A few minutes after she thus formulates her existential situation, Valerie is being carried into the night by a mysterious creature that seems to be right out of ancient Irish folklore and mythology. Her classmates stand watching, paralyzed in horror, and only Scott, armed with a slingshot, sword and shillelagh, can rescue her. It is very cinematic, as are the deaths of assorted teen-agers done in by a variety of medieval weapons, the lighting, the sets and exotic costumes. If the "Kill the Teen-agers" trend in movies lasts long enough, the film based on this book should make a lot of money.

The problem with pictures like that -- or more specifically with books that emulate pictures like that -- is that the makers thereof feel very little compulsion to explain, to motivate, to enhance the probability of the spectacular scenes that are their payoff and primary interest. It is usually not essential on the screen; uncritical viewers tend to accept poorly structured material, if it is spectacular enough to distract them, simply because they can see it happening. Witness, for example, the current success of "Time Bandits" and "Raiders of the Lost Ark." In each the plot is simply a clothesline on which to hang some technically dazzling scenes.

Perhaps there is no reason why a pastime for mass audiences should be structured like a Greek drama; you put your money into the things that sell tickets, and big scenes popping up out of nowhere sell them. But I wonder whether they will sell books. A certain level of intelligence is required to read through 300 pages, even of light fiction, and the intelligence sooner or later must focus on the holes in the plot.

John Coyne is not exactly a bad writer, though he is sometimes a careless or poorly edited one. When he is not looking toward Hollywood, he writes a decent story about life in a small-town high school, and the problems of a new boy in town who is rather shy and withdrawn and handicapped by a preppy background. He is very good at characterizing the young thugs who tend to dominate the scene in many high schools these days, and he is intriguing in his invention of "Hobgoblin," an elaborate, complex game of the "swords and sorcery" genre, based on old Irish legends and played with cards, dice and a dazzling variety of exotic characters.

The intelligent reader may wish vaguely that the game actually existed to be tried out with a few friends. The seeker of sensation may look forward to the movie, which should be satisfactorily spooky. Meanwhile, we have the book, which has splendid detail but doesn't really hold together very well. Those who enjoy well-crafted horror might do better to reread Stephen King's "The Shining."