A new "Black Case" emerges, seemingly linked to the missing Capsem once used by Code: Somnia. Acting on Zero's orders, Baku sets out to recover it before the situation spirals out of control. His search leads him to Sieg, whose fused Nightmare has fully awakened. Suspecting Sieg of stealing the Capsem, Baku transforms into Zeztz and engages him in a fierce battle, unaware that an even greater threat may be lurking behind the incident.