"BLUE EYE SAMURAI"'s first season follows Mizu's hunt for her four potential fathers, a quest that is far from straightforward. She successfully kills the first, Violet, before the series begins, and tracks her second target's gun-runner, Heiji Shindo, to the city. This leads to her confrontation with Taigen, whose honor she destroys, setting him on her trail. Her journey leads her to the fortress of the villainous Irish trader, Abijah Fowler (voiced by Kenneth Branagh), one of the four white men.
The major twist in Mizu's backstory is revealed in Episode 5: she was once married to a disgraced samurai named Mikio. For a time, she abandoned her quest and found genuine happiness, but when she revealed her martial skill, her husband felt emasculated, called her a monster, and betrayed her to bounty hunters. Her mother was killed in the ensuing conflict, and Mizu killed Mikio, cementing her belief that she is destined to be alone and that her path is one of pure vengeance.
In the finale, Fowler launches a coup against the Shogun, using advanced firearms to seize Edo Castle. Akemi, now married into the Shogun's family, uses the chaos to secure her own power, taking control of her ailing father's resources. Taigen, freed from Fowler's prison, attempts to save Akemi but is rejected by her in favor of her new ambition. Mizu infiltrates the castle and, in a climactic battle, defeats Fowler. However, just as she is about to kill him, he reveals that the remaining two white men are in London and he can lead her to them. Mizu spares him, choosing to continue her quest. The final shot shows her on a boat with a captive Fowler, sailing away from Japan and toward London, fundamentally expanding the scope of the series.