Ten-year-old Fiona and her younger brother are transported back in time to ancient Ireland where they share extraordinary adventures with the boy who claims leadership of the Fianna.
Soon after the mysterious and alluring Finn arrives at her family's home, sixteen-year-old Teagan Wylltson and her disabled brother are drawn into the battle Finn's family has fought since the thirteenth century, when Fionn MacCumhaill angered the goblin king.
The very clever Oona saves her husband, the giant Finn McCool, by outwitting Cuhullin, who seeks to prove that he is the strongest giant in the world by beating Finn.