Genetics doctoral student and longevity researcher Ryan Abernathy restricts his calories and his social life; he is terrified of death, but his life, governed by study and exercise, is hollow. The longevity study that he works for receives a tip from an Irish doctor regarding two elderly twin sisters, Cleona and Catherine Owen, who live on a remote island off the western coast of Ireland. Ryan volunteers to investigate, hoping to get DNA samples from the women in his ambitious effort to increase lifespan and fight age-related diseases and just possibly, cure death itself.

On the island, Ryan meets Cleona's beautiful yet child-like great-granddaughter, Aisling, who cares for the elderly women but is able to offer Ryan no official age documentation for them. Aisling, lonely for companionship and exhausted from the care-taking of the sick women, draws Ryan into her world by telling him a story that she learned when she was young: that she and her relatives are the final known descendants of an old island clan that, through thousands of years of inbreeding and isolation, managed to achieve remarkable longevity. Ryan doesn't believe Aisling, but by asking questions of the bar man at the local pub and by stumbling across a time-worn gravestone in the old cemetery, he becomes more curious about the unusual family. As his curiosity increases, so does that of sick Irish nun Sister Ignatius, who believes that she recognizes Catherine as a nun who disappeared from her convent sixty years ago. Both scientist and nun must grapple with their own deep-seated beliefs in order to determine the truth about the people, both past and present, of the Celtic island.