Baku meets Minami, who has become an agent, in a dream. Baku helps Minami with her mission to find a lost cat, but they are attacked by a Nightmare, and he wakes up. A "Black Case" has occurred in which Minami has grown a cat's tail. When Baku falls asleep again, he challenges the Nightmare to battle in order to save Minami, but...!

Minami is a CODE agent... Upon learning the truth, Baku is disappointed and furious, realizing that Minami had merely been "pretending to be family." In his dream, Minami is transformed into the Cat Nightmare once again. Baku transforms into Zeztz Catastrom and, despite knowing her true identity, attacks the Cat Nightmare...!?

"You will be eliminated by CODE..." After receiving this ruthless verdict from Zero, Baku infiltrates Zero's dream. Attacked by a mysterious black Zeztz, Baku is forced to confront a terrifying truth! Meanwhile, Sieg, plotting to use Nem, lures her into the Nightmare Prison. Terrified by countless Nightmares, Nem is cornered by Sieg...!?

Zero, his biological father, had been using his son, Baku, for Zeztz's sake. Unable to accept his own cruel fate, Baku is tormented by his inability to understand Zero’s true intentions. Will Baku be able to overcome this nightmare…!?

Sieg turns Nem’s nightmare into a prophetic vision, causing the Nightmare to manifest in the real world and attack civilians; witnessing the chaos, Baku uses his damaged Zeztz Driver and attempts to transform with a Capsem, but something goes wrong as he struggles to become Zeztz.

Revived, Baku enters the depths of Sieg’s psyche—driven by an obsession with Nem’s nightmares—and uncovers the truth behind his past; after identifying the Nightmare within him, Baku confronts Sieg once more, leading to a decisive battle between Zeztz and Dawn.

When Baku awoke from an ominous dream, the real world had been completely transformed. Determined to uncover the truth, Baku returns to the dream world, only to find that Nem was being targeted by Six. As Baku scrambles to protect Nem, Three appears and demonstrates a terrifying power...!?

After being saved by Nox, Baku is pressed to reveal the location of CODE's base, which he saw in a prophetic dream. It seems Nox intends to storm CODE's headquarters... After much internal struggle, Baku also resolves to destroy the activation device of Code: Somnia. Nox clashes with Three, while Baku faces off against Five and take back the world from CODE!