Castro-haters plot to nuke Fidel in this less-than-explosive suspenser. They also plan to pulverize along with him hundreds of thousands of others, employing as their weapon a lost nuclear warhead left behind by the Russians in 1962. Self-styled ""Bravos,"" the terrorists are Cuban exiles fixated on revenge. The plan is to detonate the bomb in mid-Havana on January l, 1999, the 40th anniversary of the Cuban revolution, and the fact that a good many of those blown to bits with Castro will he babies doesn't rate a Bravo eye blink. For a while, only one man stands in their way. The unlikely stopgap is an American historian named McLemore, in Cuba on a research grant. In a desultory way he's re-examining the Cuban missile crisis, hoping for an overlooked nugget to revive an academic career in decline. Enter the beautiful and enigmatic Trinidad. Ostensibly, she's a government guide. Actually, she's a highly placed member of Cuban intelligence. In ways ever available to the beautiful and enigmatic heroines of suspensefiction, she galvanizes McLemore, converting him from a burnt-out case to a live-wire. R's McLemore who works it out that the Bravos have latched onto the nuke. It's McLemore who locates the whereabouts of the missing warhead. And it's McLemore who manages to convince the CIA and Cuban intelligence that after years of double-crossing each other they have much to gain from playing it straight. When at length McLemore and Trinidad yield to their mutual passion, few readers will be surprised. Nor will they be taken unawares when in the last nanosecond of countdown, the good guys catch up with the bad guys and neutralize the nuke. Blackthorn, we're told, is the pseudonym for ""an internationally known political figure."" And he does seem comfortable with his backgrounds. What he lacks is the savvy storyteller's abililty to invent people who can make the backgrounds come alive.