The film is an animated prequel, set 250 years before the events of The Lord of the Rings and the War of the Ring. It focuses on the history of the kingdom of Rohan and its legendary ninth king, Helm Hammerhand, during a brutal period of prolonged conflict and siege.

The main narrative is told from the perspective of Héra, the daughter of Helm Hammerhand. Héra is an intelligent and fiercely independent royal figure who must rise to the challenge of leadership when her kingdom faces total destruction.

The central conflict is the devastating feud between the people of Rohan and the ruthless clan of Dunlendings, led by the charismatic but vengeful chieftain Wulf. Wulf is driven by deep resentment over the loss of territory and the perceived betrayal by the Rohirrim kings years earlier.

The initial peace treaty between Rohan and the Dunlendings collapses catastrophically. Wulf launches a massive, organized invasion of Rohan. King Helm Hammerhand, known for his tremendous physical strength and unwavering stubbornness, initially relies on brute force and traditional tactics, refusing to retreat or negotiate.

The Dunlendings swiftly conquer much of Rohan, forcing Helm and his remaining forces including Héra and her two younger brothers to retreat and seek refuge in the mountain fortress known as the Hornburg (later to become Helm's Deep).

The middle act details the long and brutal siege of the Hornburg. Conditions rapidly deteriorate inside the fortress due to dwindling supplies and continuous assaults by Wulf’s forces. During this time, Héra steps forward, offering strategic wisdom and demonstrating courage far exceeding the traditional expectations for a princess.

Tragedy strikes as the siege continues: Helm Hammerhand loses both of his sons in battle. Devastated and consumed by rage and grief, Helm descends into a legendary madness. He begins conducting solitary, ghost-like raids out from the Hornburg during the deepest cold of winter, striking fear into the Dunlendings by using nothing but his bare hands as weapons.

The film’s climax is set during the harshest winter. With the Rohirrim on the brink of starvation and surrender, Héra must take decisive action. She organizes a desperate mission to seek aid from Rohan's scattered allies, riding out of the Hornburg through the besieged passes. Concurrently, the weakened Helm fights his final, fatal confrontation against Wulf in the frozen ravine outside the fortress.

The story concludes with Héra successfully returning with reinforcements, breaking the siege and driving the Dunlendings out of Rohan. Though King Helm Hammerhand has perished in the fight giving his name forever to the fortress and the Deep Héra ensures the survival of her line and the legacy of her people, securing Rohan's future by demonstrating true leadership through intellect and resilience.