By Peter Straub

Random House. 482 pp. $25.95

Who is Mr. X? That's the mystery for most of Peter Straub's book, a long, drawn-out affair that takes stabs at horror, love story, legal thriller and whodunit.

Mr. X is many things at once: He's a beatnik, a crime lord and a monster with a few supernatural tricks up his sleeve. He's also a writer--an obsessed H.P. Lovecraft fan who writes ranting, otherworldly journals that would make Vincent Price wince at the campiness of it all.

When we first meet this X-man, he's in the business of killing people for no discernible reason. His real goal in life, however, is to find his son and separate the little bastard from his mortal coil, an act that he believes will fulfill some sort of dark prophecy. Ah, those confounded dark prophecies!

Meanwhile, we meet software programmer Ned Dunstan, a college dropout who was brought up in a series of foster homes and has always felt that something was missing from his life. He comes home, pulled by a psychic sense that seems to run in the Dunstan family, back to Edgerton, Ill., where his mother, Star Dunstan, a nightclub singer and aging wild child, is on her deathbed. When Star whispers the name of his father before passing away, Ned stays in town to try to learn a little family history and find out about the man.

Can anybody spot where these two plot lines might converge?

It turns out that Star gave up her son thirty-some years ago not just because she was a free spirit and not the mothering kind but also because she wanted Ned to have a shot at a normal life, something he could not have had in Edgerton. You see, there are two major families in Edgerton. There are the Hatches, the rich folks who own the place, and the Dunstans, who have something of a bad rep, starting with Omar and Sylvan, a creepy pair of twins who rolled into town back in 1874. Nobody knows where they came from, but horrific rumors tended to follow them around, such as the one about how some of their babies came from the womb hideously disfigured and the corollary rumor that these deformed monsters are being kept up in the attic.

There are a number of surprises waiting for Ned in the old burg. For one, he discovers he has a brother, a shadowy doppelganger named Robert who drifts in and out of corporeal form without much explanation. Robert's existence has been kept a secret from him--again, to protect the fellow. Like a few other characters, Robert alternates between seeming like a threat and seeming to be a good guy. Eventually he disappears for good.

Ned also discovers that he has the power to hop into the past and escape the present. This little talent isn't really accounted for--nobody else in this spooky family seems to have it--but seems to exist for no other reason than to let Ned conveniently escape the occasional plot jam.

In a horror tale, perhaps it shouldn't matter so much if you can guess how things are going to wind up; just getting there and being jolted by surprises on the way should be half the fun. But Straub so consistently throws red herrings in the path of the story line that following the plot becomes an obstacle course. Characters are hastily introduced, then hang around only to behave with maddening inconsistency.

And what of the title character? Not to give any more away, but it turns out that, among his other roles, our Mr. X was also the victim of an elaborate and long-running practical joke. Yes, he's pretty bad, mind you, and yes, he has supernatural powers--but by the end it turns out that the guy is also a loser, the sort of guy you'd avoid at a party, not because he's such a very bad man but because he's dull and slightly unhinged. His real identity doesn't come as a surprise or a disappointment; it just sits there.

The writing in "Mr. X" isn't particularly a source of pleasure, either. The dialogue is wooden, and Straub has a habit of rolling out phrases that are impressively nondescriptive: "Laurie reassembled herself without altering her posture or moving any part of her body." Picture that, if you will. I can't. In another scene, one Stewart Hatch is described as having "the beveled haircut of an untrustworthy senator."

By throwing out lines like that, and by making Mr. X the Diet Coke of evil (to borrow a line from "Austin Powers 2"), Straub undermines what mood of horror he has managed to build. For that matter, the reader never does learn where Omar and Sylvan, the original Dunstan duo, came from or how this dysfunctional family line came to be in the first place. The scariest thing about this book is what a confusing mess it is.

Mike Musgrove, who is on the staff of The Post's Fast Forward section.