The Eightfold Fence

Toranaga, Blackthorne, and Mariko arrive at Ajiro, and Mariko has Fuji to serve as Blackthorne's consort, despite her reluctance. Blackthorne finds the Erasmus and attempts to sail in a boat with help from Muraji, only to be stopped by Mariko, who informs him that his ship has been confiscated and now belongs to Toranaga, and his men have been sent to an unspecified location. Toranaga converses with the villagers before leaving for Edo.

Blackthorne is given a home and provided with a good salary. He starts training Yabushige's army, although he struggles due to his lack of wartime experience. Eventually, he decides to use his maritime knowledge to teach them how to use cannons, impressing Yabushige. Blackthorne also begins to learn more about Japan's culture and slowly learns the language. Ishido's retainer Nebara Jozen arrives and tells Yabushige to return to Osaka and pledge his loyalty to the remaining regents. When Yabushige tries to deviate, Jozen warns him that he will face treason if he refuses. To ease tensions, Yabushige invites Jozen to stay the night and witness a demonstration of the cannons the next day.

Grateful for her services, Blackthorne gives Fuji one of his guns as a token of gratitude, and she gives him her late father's swords. He also goes to swim and has a conversation with Mariko on his life back in England. She later visits him in his room and they have sex, although later she pretends to have hired a courtesan with help from Fuji. At the demonstration, Toranaga's son Nagakado deviates from the plans and leads an ambush on Jozen and his men, using the cannons to kill several of them while also blowing off Jozen's arm and killing his horse. Yabushige protests about this as it will mean an unprovoked declaration of war, but Nagakado tells him to inform Toranaga as his men finish off the survivors. A badly wounded Jozen angrily denounces the use of the cannons and calls Nagakado's actions savage, only for Nagakado to behead him, much to Yabushige's horror. A horrified Mariko says that war is now inevitable.

In February 2024, Hulu confirmed that the fourth episode of the series would be titled "The Eightfold Fence", and was to be written by Nigel Williams and Emily Yoshida, and directed by Frederick E. O. Toye.^[1] It was Williams' first writing credit, Yoshida's first writing credit, and Toye's first directing credit.^[2]

The episode is dedicated to the memory of Larry Beckett, the series' technical advisor and cannon expert.^[3]

The final scene deviates from the original novel. While Blackthorne teaches the army and Nagakado kills Jozen, the novel depicts Blackthorne showing the use of muskets, while the series employ chain shots from the cannons. Justin Marks and Rachel Kondo explained that the change was because it was more historically accurate, as Japan had already been introduced to muskets 50 years prior to the events of the story.^[4]

Marks said that Nagakado was in a "tough and not-so-tough position", citing his ambitions in living up to his father's standards, and suggesting he could've ended like Omi had he not been born in his lineage.^{[5]: 4:10–5:37}