Mr. Apple is one of the most engrossing characters in recent fiction. A former diplomat, he now runs a bookie shop in Belfast. Legitimate as far as it goes, the shop also serves as a conduit for laundering Irish Republican Army funds. Besides making book and washing money, Mr. Apple practices double agency.

None of this, however, is really what makes him so fascinating. The essence of his appeal lies in the contrast between the placid persona he offers the world and the roiling psyche he keeps hidden away. In his shop and on the streets, Mr. Apple is an unflappable, passive-aggressive shadow, whose favorite trick is to reduce an inquisitive customer -- who most probably is a spy from one side or the other dispatched to ferret out Mr. Apple's true allegiance -- to discomfort with a few well-aimed quips and strategic silences.

While serving as a diplomat in Mexico, Mr. Apple underwent a strange (and never quite explained) ordeal that culminated in his torture and the infliction of multiple wounds that still occasionally bleed. (You can't have a novel about Northern Ireland without a little lurid religious imagery.) Worse, it left him prey to horrid nightmares and uncanny premonitions. As M.S. Power's taut, masterly novel "The Killing of Yesterday's Children" opens, Mr. Apple's premonitions center on the mingling of his destiny with that of Martin Deeley, the professional killer the IRA has stationed in the shop, ostensibly as Mr. Apple's assistant, in fact as their snoop.

Martin's excuse for killing is that the Protestants did the same to his father and brother. Once that happened he "knew he would be expected to join the unending cycle of violence and avenge these two helpless deaths. Oh, yes, Mam would see to that, making it clear she would be ashamed of him if he did nothing about it . . . And friends would argue knowingly that two deaths in one family were enough, but their arguments would be couched in words that expected rebuttal . . ."

The plain truth, however, is that Martin digs killing. He revels in his marksmanship, and scenes like the following give him pleasure: "Even, it seemed, before the brittle crack of the rifle reached his ears he saw the man hurtle several feet backward, saw him attempt to swim, his arms and legs flailing in a grotesque backstroke, saw him shudder and lie still."

Feigning sickness, Martin takes off one afternoon. That night Mr. Apple hears on the news that someone has tried to assassinate an English army officer stationed in Ulster but just then home on leave in England, missing him only because the chauffeur unwittingly moved into the bullet's path. The next day Mr. Apple asks Martin if he wasn't the gunman. Martin admits he was. Both men suddenly realize they have dropped the tension-filled banter that had dominated their conversations until now. On an impulse Mr. Apple offers Martin his friendship. Moved as profoundly as he has ever been in his life, Martin accepts the offer. When it becomes clear that the IRA is going to kill Martin and throw his corpse to the incensed British in an effort at damage control, Martin finds himself in great need of friends.

Besides Mr. Apple there are an IRA informant whose life Martin once saved and Daphne Cole, a prostitute with a "nicotine-stained voice" who loves him like a faithful pup. Together they all contrive a plan for foiling the IRA and spiriting Martin and Daphne out of the country.

This is a highly promising plot. Yet in the end Power lets the reader down a bit on the narrative end of things, largely through the miscalculation of presenting a table-turning development offstage.

But this hardly matters. Power's characterizations are so skillful and his prose so exact and inventive that the outcome is secondary to the pleasure of reaching it.

As for Mr. Apple, I don't mind telling you his Christian name -- Arthur. But he is so real to me that if I heard anyone call him anything but Mr. Apple, I think I would wince.