In 1578, Araki Murashige, Masahiro Motoki the merciful lord of Settsu, rebels against the brutal warlord Oda Nobunaga, barricading himself inside Arioka Castle as Nobunaga lays siege. Murashige's retainers await reinforcements from Mōri Terumoto in Harima Province. Oda sends an envoy in the form of the cunning and intelligent military strategist Kanbei Kuroda, Masaki Suda, who tries to convince Murashige to abandon his unwinnable rebellion; Murashige retaliates by throwing him in the dungeon.
As winter settles, the lord of Ōwada Castle, Abe Jūemon, defects to Nobunaga. His son, Abe Jinen, is kept as a hostage in Arioka Castle, and it is customary for such hostages to be crucified. Jinen, a devout Shinshū Buddhist, pleads to be killed by a sword to earn entry into Paradise, but Murashige instead imprisons him. Some time later, Jinen is found dead, seemingly from an arrow, yet no arrow is found on his body. Murashige gathers testimonies and brings them to Kanbei, who reads an obscure poem in the documents. The poem’s meaning leads Murashige to identify the killer, whom he spares but sends into battle, where the killer is killed. Kanbei, Masaki Suda, helps piece together the truth through careful deduction, revealing how perception and fate intertwine in the siege.
In spring, one of Murashige’s retainers, Ichirōza, discovers an army led by the young commander Ōtsu Nagamasa preparing to attack in an effort to impress Nobunaga. Murashige ambushes the army, aided by the Buddhist Saika Clan under Saika Magoroku and the Christian Takatsuki Clan under Takayama Dario. A barely-clothed swordsman named Hori Yatarō attacks Murashige’s camp, killing Ichirōza before being slain himself. When the heads of the generals are inspected, both the Saika and Takatsuki clans have taken two heads, each including the head of a young commander. Murashige gathers the clan heads to determine who killed Nagamasa. The heads are switched—one of the young commander’s heads is swapped for Hori Yatarō’s—injecting fear into the villagers. The tension between the clans escalates, culminating in the burning of the local Christian church. Murashige seeks guidance again from Kanbei, who hints that Nagamasa’s head was not taken at all; Nagamasa wore a helmet, revealing him as a commander, while Murashige himself killed Nagamasa, resolving the clan dispute.
In summer, Murashige discusses a potential surrender to Nobunaga that would allow him to regroup through a liaison with Nobunaga’s retainer Akechi Mitsuhide. Mitsuhide requests proof of Murashige’s willingness to surrender in the form of the valuable tea urn Tora-saru, which Murashige entrusts to his envoy, the Buddhist priest Muhen. Before leaving Arioka, Muhen is found dead in his lodgings, along with a skilled swordsman, Akioka Sironosuke. A search of the lodgings reveals that the letter to Mitsuhide was discovered but not taken, and Tora-saru has been stolen. Kanbei’s relative, Zensuke, infiltrates Arioka and reveals that Kanbei’s son Shōjumaru has been killed by Oda, who suspects Kanbei of defection. Murashige forgives Kanbei but unshackles him from his chains and offers him a role on the war council, though Kanbei declines, citing that Settsu’s samurai would not accept a Hariman-born leader, and thus remains in the dungeon. Through Kanbei’s deduction, Murashige identifies Notō, a proud retainer, as Muhen’s killer. Notō is chased to the ramparts, where Murashige accuses him of treachery toward Nobunaga; Notō dies in the confrontation, struck by lightning as he attempts to rise again. With Tora-saru recovered, Murashige proclaims that he will stop using worldly treasures as tools of war and will instead treasure them for peace.
In autumn, with faith waning among his vassals and Mōri’s promised help not arriving, Murashige works to uncover the web of plots against him. With Kanbei’s help, he uncovers that his wife, Lady Chiyoho, has orchestrated much of the misfortune: she ordered Jinen’s death, arranged the head-switching plots, and caused Notō’s death to demonstrate that divine punishment could strike anyone. She cites her trauma from the Nagashima siege, where the Ikkō-ikki believed retreat led to damnation and charging into battle led to Paradise, using divine punishment to teach a lesson. Murashige returns to Kanbei in the dungeon and, over a quiet drink, is advised to seek a route to Mōri by sea. Murashige, initially buoyed by the plan, realizes that Kanbei has been orchestrating his downfall for months and fears public disgrace if he abandons Arioka. Nevertheless, Murashige chooses to follow the plan, instructing his followers to surrender peacefully if the castle falls and to claim that he has fled. He sets out on the journey to Harima, with his entourage dwindling; one by one, his companions choose to return to Arioka, pledging loyalty to the castle rather than to their lord. Only Sukesaburo remains steadfast as Murashige’s lone companion on the long, uncertain road.
The final shot carries an epilogue: Nobunaga does capture Arioka Castle while Murashige is away, yet Murashige survives and becomes a renowned tea master. The narration notes that Nobunaga would later fall three years after Arioka’s siege due to a rebellion led by Akechi Mitsuhide, known as the Honnō-ji Incident, while Murashige’s own life takes a divergent path, marked by a lasting legacy in tea culture and restraint.
