Recently widowed Conor McGuile acquires an antique railcar which he restores and outfits with all the comforts of home. He learns the caboose is haunted by an Indian warrior unjustly accused of robbing a bank courier of a fortune in gold. Conor determines to right the wrong and restore the brave's reputation.
Accompanied by his best friend Néall, the pair set sail for New York from Ireland aboard Conor's Atchison, Topeka and Santa Fe caboose. Conor intends to visit his grandson Cormac (Mack) in the American Southwest, in New Mexico. The journey across the Atlantic Ocean takes a harrowing two months, during which time the friends land on a fantastic mid-ocean seamount. The sole inhabitant of The Unknown Island reveals astounding secrets and incredible coincidences that will affect Conor and Néall for years to come.
Once in the New World, Néall, reconciled with his wife, returns home to Derry, Northern Ireland, while McGuile travels across America. He encounters a trainload of interesting characters, from a runaway bride who reminds Conor of his deceased wife to a ten-year-old boy who wants to adopt the old man as his grandfather.
Conor's grandson and his companion, Pedro, abandoned by their parents as teenagers, live in a dilapidated adobe house in Cold Beer, New Mexico. Conor spends a good while disciplining the boys and repairing their tumbledown abode. At last, convinced that the young men are "flying straight," McGuile, homesick, returns to Ireland where the final pieces of his life fall neatly into place and he realizes the beneficial effects he has had upon all whom he met.