Back home in San Francisco after a year in Dublin, computer engineer Niall Sweeney enjoys a pleasant routine working in a bank and fiddling at night in an Irish bar. Suddenly, he is the target of a vicious and insane bar-room assault at the hands of a near-stranger. But Sweeney's troubles really begin when he discovers the mutilated body of his attacker outside, opened like a can of sardines.
Horrifying coincidences crash down on Sweeney. Baffled, he is fingered by the police as the link connecting the murder with a complex IRA banking scam and a deadly London bombing. With the help of his wife Rose, a brilliant college literature professor, Sweeney tries to keep one step ahead of both the police and a malevolent shadowy stranger while searching desperately for the truth.
While Sweeney hunts among the bar musicians for the murderer, discovering only smoldering fear, secret affairs and a bartender who is far from what he seems to be, Rose launches off in another, seemingly crazy direction, convinced that the secret can be found in the murdered man's obsession with a haunting thousand-year-old Irish poem-The Legend of Mad Sweeney.
Rose is first to stumble upon the bizarre truth. A tale of international complexity, cruelty and coincidence unfolds - a tale of a paid IRA informer's flight from retribution, a brutal rape, multiple identities, twisted Catholic guilt, not one but two murder weapons, and... the impossible clue that connects it all... the not-so-mythical Mad Sweeney himself.
Rose learns, to her horror, that by revealing the strange solution to the mystery she would destroy the lives of half a dozen innocent people. The final decision about what to do is made for her by the sinister shadow man in a terrifying and deadly climax. But to save her husband, Rose must give the police a solution to the murder that is as believable as it is completely wrong.