When Renais is suddenly invaded by its erstwhile ally, the empire of Grado, Eirika flees under escort to the kingdom of Frelia.
She soon learns that Grado's objective is to collect the five Sacred Stones that once sealed away the Demon King Fomortiis, and sets out to protect the remaining stones by working with the nations that hold them. Later in her journey she reunites with her beloved twin brother, Ephraim, who had boldly charged ahead into Grado's territory at the start of the conflict.
With the Sacred Twin relics of Renais—the Thunder Blade Sieglinde and the Flame Lance Siegmund—they combine their efforts and overcome Fomortiis together.