Years ago Danny Carmody and Willie Dowd,two childhood friends from the slums of Belfast, served in the Provisional wing of the Irish Republican Army. Dowd, slight and studious, gave himself without reservation to the cause and rose through the ranks by his dedication, intelligence, and talent for making bombs. Carmody, a big lad and far from intellectual, joined the Provos not from any political conviction, but out of loyalty to Dowd, following a pattern he had held to all his life.

Together they had planted bombs, staged ambushes and robbed banks in the name of the cause, each risking his life to save the other. But when Dowd forced Danny into a shameful act, Danny felt he had been betrayed. He quit the Provos and fled to America, nursing a bitter hatred of Dowd for what he saw as the wasting of his life. Burned out and plagued by guilt for the destruction and deaths he had caused, Danny drifts along the Eastern seaboard, winding up an unofficial bouncer in a black bar in Newark, New Jersey. He has changed his name to Milo Costigan.

In the meantime, Dowd pursued his IRA career, wounded by his friend's desertion, but never once questioning the cause or his place in it. When a cease-fire begins, Dowd finds himself a man without a function. His uncompromising dedication and fanatic hatred of the British are no longer wanted. At the urging of a friend, he comes to America. His friend Terry has discovered some shady dealings at the plant where he works. Terry has a plan for hijacking evidence of the plant's criminal activities, and demanding a million dollars ransom.

The plan goes through and they send a ransom demand to the head of the conglomerate that owns the plant, a ruthless and calculating billionaire named Jason Ackerdyne. Ackerdyne wants the situation dealt with and Dowd eliminated without involving the police. He gives the problem to his chief of security Julia Malcom. Malcom, a devious and fiercely ambitious woman, finds Milo Costigan, (the former Danny Carmody), and uses Costigan's hatred of Dowd to manipulate him into taking the job, without, however, telling him the whole story.

The Lonely Impulse is a novel of a little under 70,000 words that tells what happens as Dowd tries to get his ransom, and Milo Costigan tries to hunt him down.