One bright morning, the villagers near Blarney Castle, Ireland hear terrible news: the famed Blarney Stone has been stolen. Enter Joe Mulqueen, singing insurance investigator from New York. The lovely daughter of police sergeant McNaughton soon catches Joe's eye, and oddly enough Joe fulfills an ancient prophecy of who her lover will be. Meanwhile, Joe does find time to do a little investigating...

In a small town near Cork, Ireland, civic guard Sergeant Briany McNaughton alerts his assistant, Hughie Devine, that the famous Blarney Stone has been stolen from the castle. This marks a big day for the village, as there has been no comparable crime in over thirty years. When Briany and Hughie investigate, they learn that villager Cormac Gillespie was seen near the castle the night before. Chief Inspector Fallon, from the Irish police force, orders Briany off the case because he does not consider him a professional, but Briany nevertheless determines to solve the crime. Meanwhile, in New York City, insurance investigator Joe Mulqueen is assigned to work undercover to find the Blarney Stone because his parents were Irish and his company insured the Stone for $500,000. Joe spends his first day in Ireland near Blarney Castle posing as a painter, and is befriended by some boys, who introduce him to Briany's beautiful daughter Conn, as she washes clothes in the castle's stream. Conn is stunned by Joe's presence, as he seems to fulfill a well-known prophecy about the theft of the Stone. However, suspicious Hughie arrests Joe and brings him to Briany for questioning. Although they lock Joe up when he refuses to give them any information about why he is there, they release him after he wins them over by singing an Irish ballad. Although Briany considers Joe a prime suspect, he invites him to his house for a party. That night at the party, Conn consults with wise woman Biddy O'Devlin, who is the keeper of the Stone's legend, and confirms that so far, Joe has fulfilled the conditions of the prediction, down to the coat he is wearing that night, and the fact that Conn is falling in love with him. When Joe returns from the party, he finds Briany and Hughie searching his room, and they learn that Joe is an investigator and hear his dictaphone recording, in which he says that Briany is not effective. Briany considers throwing Joe out of town but resists when he hears about the insurance company's $5,000 reward. Although everyone suspects that Cormac is the thief, Fallon tells Joe that the only fingerprints found on the castle wall belong to Hughie. Joe becomes more suspicious of the deputy when Hughie shows little remorse after his only relative dies in a cart accident. Joe starts to link together the accidents and recent unusual events that have been blamed on the theft, and formulates a secret plan with Fallon. One night, Joe and Conn enjoy their first kiss, but their joy is crushed when Briany arrives home with news that Fallon has fired him because Joe intimated that he might impede their investigation. Conn throws Joe out because he deceived her about his work, and because she hoped her father would find the Stone in order to restore his pride. Biddy warns her that the predictions indicate that the stranger of legend will be killed in the Killeen forest if his loved one abandons him. The next day, Joe investigates the site where Hughie's cousin died and realizes from tracks in the ground that someone tipped the cart over while it was stationary. Joe insists that Briany be reinstated, and arranges to meet Fallon and Briany that night at Biddy's cabin in the Killeen woods. Biddy then tells the police that the Stone was stolen to cover up the murder of Hughie's cousin. Everyone deserts Biddy to chase Cormac when he suddenly appears, but Hughie is disturbed by a comment Biddy made about his soul, and knocks out the boy sentry, Pearse O'Neill, to get her alone. Hughie is about to strangle Biddy when Joe and Briany stop him, and he reveals that he murdered his cousin for the money. Hughie is arrested, and before he is taken away, he vows never to reveal where he hid the Stone. Biddy realizes that although Joe did not die, the final prediction has come true, as it says that the singer, who is Pearse, will be struck down by violence, and the song will not be finished by a man. Pearse revives and everyone wanders away from Biddy's cabin, taking a path past a 700-year-old chapel. When Briany sits down upon a large stone that moves, Conn marvels that a stone set so long ago would suddenly come loose. Joe realizes that Briany has discovered the Blarney Stone, and Briany is now hailed as a hero.