It's the Warring States period. Many a warrior dream of unifying Japan. Toyotomi Hideyoshi, on the verge of realizing that dream, takes a massive army led by Ishida Mitsunari to surround Odawara Castle, currently being defended by Hojo Ujiyasu.

The death of Hideyoshi Toyotomi, ruler of Japan, plunges the world into turmoil yet again. Mitsunari Ishida tries to protect the Toyotomi clan by opposing Tokugawa, the strongest faction, but his behavior only causes misunderstandings and further isolates him.

Having learned of Mitsunari Ishida's taking up arms, Ieyasu Tokugawa decides to march his massive army to take him on at Sekigahara. Nobuyuki Sanada, who had parted with his brother Yukimura in the lead-up to this event, would have to join the fray as part of a unit separate from Ieyasu's, and under the control of his son Hidetada.

While the Sanada brothers Nobuyuki and Yukimura face off at Ueda Castle, and Kagekatsu Uesugi and Kanetsugu Naoe take on Masamune Date and Kojuro Katakura at Aizu, the curtain is drawn on the Battle of Sekigahara. With the overwhelming numerical advantage created by Yukimura's stalling of Hidetada's forces and a formation that fully utilized his geographical superiority, Mitsunari takes the upper hand in the early going.

Mitsunari Ishida was killed in the Battle of Sekigahara. Following the war, a meeting is held at Edo Castle to determine the fates of those that had stood against the Tokugawa clan. Yukimura Sanada's sentence, in particular, attracts heated debate from all those present. Amidst all the voices baying for his blood, not least Hidetada Tokugawa, who had fallen in battle to Yukimura, and was late in reaching Sekigahara as a result, Ieyasu Tokugawa asks Yukimura's brother, Nobuyuki, for his thoughts.

As the Tokugawa clan's messenger, Nobuyuki heads to Aizu to inform the Uesugi clan of their punishment following the battle of Sekigahara. He meets Kanetsugu Naoe, who refuses to accept the severe, and almost cruel, demands, but Nobuyuki refuses to budge too. On the other hand, while Yukimura escaped the death penalty, he was leading an empty life, without being able to find the "meaning of his life" that Nobuyuki had asked him to reconsider. But one night, things start to get noisy in his vicinity.

Hidetada Tokugawa succeeds his father as the second Tokugawa shogun, and within heightened anti-Tokugawa sentiments at Osaka, Nobuyuki Sanada heads to Osaka Castle in order to meet Hideyori Toyotomi. At the same time, Yukimura Sanada's life under even stricter supervision gets a jolt from an unexpected visitor.

Having realized that he can only live as a warrior, Yukimura Sanada escapes his confines at Kudoyama and heads of Osaka. At the same time, Kiyomasa Kato and Masanori Fukushima cut ties with the Tokugawa clan and also head for Osaka.

When Yukimura Sanada and Keiji Maeda reach Osaka after a long journey, they are ushered into the castle for a meeting with Hideyori Toyotomi. Having learned of his young master's resolve, Yukimura reaffirms his desire to fight for the Toyotomi clan.

This is the beginning of what is now known as the Winter Siege of Osaka. The Tokugawa army tries to assert its numerical advantage, but Osaka Castle and the Sanada Maru fortification stand firm against them. Yukimura Sanda, Keiji Maeda, Kiyomasa Kato, and Masanori Fukushima fight hard as part of the Toyotomi clan's forces, refusing to give an inch to their enemies. With winter upon them, a cold wave saps both armies of their energy. That's when Yukimura has to face off with Kanetsugu Naoe, who he had shared a vow of righteousness with, as an enemy. Meanwhile, as Osaka Castle continues to get bombarded by artillery fire, Nobuyuki Sanada charges in alone in a bid to negotiate a truce.

When Hideyori Toyotomi accepts the Tokugawa clan's terms for the ceasefire, the Winter Siege of Osaka comes to an end. However, even the inner moat of the castle gets filled in despite not being one of the terms, pushing the Toyotomi clan further to the brink. It was clear to one and all that the last war of the Warring States Era could not be avoided. While everyone prepares for the final showdown, Nobuyuki Sanada decides to shoulder the pain of killing his brother, Yukimura. On the other hand, Yukimura tries to grasp the one chance of victory the Toyotomi clan has against all odds.