Synopsis:Llywelyn ap Gruffydd and his brothers Owain and Dafydd are heir to the Principality of Gwynedd, in North Wales. But Llywelyn seeks to unite the whole country under one leader. His brothers want a piece of the action. But Llywelyn's plans are halted by the Barons' Revolt led by Earl Simon De Montfort, who feels the King of England, Henry III, has too much power. The Earl, hoping to find an ally in Wales against Henry, approaches Llywelyn and the two become fast friends, particularly when Llywelyn falls in love with the Earl's beautiful daughter, Eleanor.
However, the Barons' Revolt is crushed at the brutal Battle of Evesham and Simon is killed and his family sent into exile. Llywelyn appeals to the new King of England, Edward the First, to allow his marriage to Eleanor to proceed, but Edward takes Eleanor prisoner as she sails for Wales. Llywelyn is forced to pledge his allegiance to the English king in return for his betrothed.
Safely back in Wales with his bride, Llywelyn vows not to fight again. However, the English nobles continue to harass the Welsh Border Lords, including Llywelyn's old friend Rhys Fychan. Llywelyn's brothers side with Rhys and Llywelyn tries hard not to be drawn into the conflict. But when Eleanor dies in childbirth Llywelyn finally yields to the demands of his countrymen and leads his army to join with his brothers and Rhys.
Refusing all English appeals to surrender, the Welsh army marches on Builth Castle, the largest English stronghold in Wales. But Edward's army awaits them in the forest at Irfon Bridge for a final showdown...