Set 183 years before the events of The Lord of the Rings, The War of the Rohirrim unfolds in the kingdom of Rohan, a land of horse-lords and rolling plains. The story centers on Helm Hammerhand, the hot-tempered King of Rohan, and his daughter Héra, a fiercely independent young woman who is uninterested in marriage and duty. The conflict ignites when a Dunlending lord named Freca arrives at Helm's hall with his son, Wulf, a childhood friend of Héra. Freca seeks to force a marriage between Wulf and Héra to usurp the throne of Rohan. When Helm refuses, a fistfight breaks out, and the king inadvertently kills Freca with a single, powerful blow, earning him the nickname Hammerhand.

Banished and devastated by his father's death, Wulf vows revenge. He disappears for years only to return as the vengeful High Lord of the Dunlendings, a host of hill-tribe rebels. Launching a sudden attack against Rohan, Wulf seeks to destroy Helm, his family, and his entire kingdom. As the situation grows desperate, Héra must step forward to lead the resistance. After a series of betrayals and devastating losses—including the deaths of her brothers—she evacuates the surviving Rohirrim to the ancient stronghold of the Hornburg, a fortress that will later become legendary as Helm's Deep.

Throughout a brutal winter siege, Helm becomes a legendary figure, sneaking out at night to strike fear into the enemy with his bare fists, eventually dying on his feet in the snow, frozen in place but unbowed. Héra, embodying the will of her people, ultimately faces Wulf in a final confrontation to end the war. The narrative is framed as a story told by Éowyn, the shieldmaiden of Rohan from the original trilogy, who narrates the legend of Helm Hammerhand and the foundation of Helm's Deep. While the men of Rohan fall or falter, it is Héra’s courage and resourcefulness that define the kingdom’s survival, honoring the tradition of epic fantasy while introducing a fresh, anime-driven perspective on Middle-earth.