TROUBLE By Michael Gilbert Harper & Row. 254 pp. $15.95

THERE ARE the makings of two good stories in Michael Gilbert's Trouble. The first involves an intricate plot by the Irish Republican Army to smuggle explosives into London for purposes of a murderous Christmas terrorist offensive against the Brits. The second is a study of the tenuous state of contemporary race relations in Britain as reflected in the escalating warfare between rival gangs of young working-class Britons and Pakistani immigrants, and the thankless, uphill efforts of the local parole officer to keep the simmering racial cauldron from coming to a boil.

One suspects that both themes, fleshed out, would stand quite nicely on their own. But when the themes are merged in this book, the result, cleverly plotted and well-crafted (as one would expect from a winner of the Mystery Writers of America Grand Master Award), is ultimately disappointing and only occasionally compelling.

Gilbert hits you with his best shot at the outset as British security forces successfully interdict an IRA cross-Channel smuggling operation, seize explosives destined for the terrorist campaign, and nearly nab Liam, the brains behind the IRA plot. The IRA exacts grisly retribution through the torture-murder of the undercover agent who compromised the operation, and the Brits gird themselves for Liam's next move. The action's fast-paced and the dialogue's a colorful mix of Elmore Leonard grit tinged with continental grace, as when an irate heavy coolly advises his captives: "If you do not, all of you, get into the car at once, I shall be regretfully forced to throw this young sprog of yours into the sea. It's a long drop and he'll bounce twice on the rocks before he reaches the water."

The escalating warfare between the Young Britons, a gang of London semi-toughs, and a rival gang of Pakistanis who push back hard when pushed, also makes good reading. Gilbert paints a grim picture of two groups on a collision course that is brutally realized.

So far, so good. Where Gilbert and Trouble ultimately stumble is in melding the two themes into a credible and cohesive whole -- to convince the reader that the elaborate plot which Liam and the IRA set in motion in a second attempt to smuggle explosives into London requires using the rival gangs as an unwitting centerpiece. Liam's scheme is clever, most of the pieces of the multifaceted puzzle click into place but, that said, the reader is left wondering why all the complexities are necessary. It's as if Liam has hired Rube Goldberg as a consultant.

As a result, the story frequently slows to a plod. Which is too bad, because when Trouble moves, it is deftly handled. Gilbert's a first-rate stylist, and leaves little to the imagination with a gripping graphic style, as exemplified by the parole officer Leone's reaction the night the racial cauldron boils: "A moment of blinding light, a great flash of flame swallowed by a wash of blackness. The sound and the shock wave hit him together. Sight and thought were suspended. Then he saw the orange and crimson glow of fire and started in a stumbling run, unhurt, but dazed, down Wick Lane."

Gilbert's major characters are credible. Liam's one appropriately tough customer displays a professionalism matched only by his amoral cynicism (leading a colleague to remark "Your knowledge of the underside of human nature is a perpetual source of amazement to me.") His hard-boiled adversary, Colonel Every, doesn't let sentiment get in the way of his pursuit of Liam and the explosives. Advised of recent developments involving the gangs, Every observes dispassionately: "Either the Pakis blew the Brits up, or the Brits made a mistake and blew themselves up. It doesn't affect our problem."

What does affect Gilbert's problem is the fact that Liam's plot, shorn of its gratuitous intricacies, doesn't lend itself to piecemeal discovery and disclosure. As a result, Gilbert has to hide salient facts to shield the outcome, then fill them in with a rush afterward. The conclusion reads like the closing minutes of a Perry Mason episode where Raymond Burr sits down with Della and Paul Drake and fills in all the gaps a` la Vanna White.

At bottom, Gilbert has crafted some entertaining individual gems (the two London cops who confront a child pornographer in his little shop of horrors make Starsky and Hutch look like candidates for ACLU membership), but which strung together yield a slender necklace of a story. It's a case where the parts, for all their sparkle, are greater than the sum. And that's the trouble with Trouble.

Rory Quirk, a Washington attorney, is a frequent contributor to Book World.