In many ways the Prison arc is the pinnacle of Hiroaki Samura's attempt to push Blade of the Immortal past mere samurai action into the realm of pure art. From its anti-dramatic structure to its role reversals (Rin does the rescuing) and increasingly sophisticated pencil art, his ambition informs every aspect of the story. The punked-out excess of early Immortal has been traded in for careful, Kurosawa-styled humanism and political insight, and the plot has the simplicity of a darkly relevant fable. Even as the arc delves deep into action territory, its fights remain rooted in the intricacies and strategies of real-life swordplay in a way that recalls the artful minimalism and studied realism of Lone Wolf and Cub as much as it does the gonzo bloodletting of early Immortal.
You can almost taste the series' thirst for respectability. But the obviousness of the series' intent doesn't lessen its effectiveness. The Immortal on display here is controlled and insightful. Its slow-burning narrative heats incrementally to a raging climax, even as it explores timely ideas about medical testing and the military application of medical technology. Its vision of the horrors unleashed by a government that has gained possession of a living key to immortality is frighteningly plausible and thoroughly nauseating. This is probably the most focused and mature the series has ever been.