This is a world where beastmen, dragon-kin, and demi-humans exist. In this near-future world, a distinct threat exists that differs from our modern reality: monsters known as "Bugs" that seep in from parallel universes. Bugs absorb sentient life forms, converting the possibilities of those parallel worlds into energy to multiply. Once they reach a certain threshold, they transform into "Errors," a direct threat to humanity. Humanity struggled to find a way to counter this threat, but their efforts were largely unsuccessful. However, a scientist developed artificial life forms embedded with a "Core System." Beast-type Driven Parallel Action Units. Commonly known as Beast-War Drive-Units. They possess a unique drive system in their lower abdomen called a Parallel Coupling Reactor, allowing them to exert combat power far exceeding that of an entire army. They are combat units specifically designed to counter Bugs and Errors. These units possess clear individual wills. When the Core System is activated, someone in the world is registered as their "Partner." This partner is called a "Driver." While protected by the state, their free will is respected. The military's Special Mobile Task Force, the Special Beast Agency—also known as "Firewall"—was established to manage and operate them.

Synopsis Due to the defection of Beast-War Drive-Unit No.3, Repro-Tyga, a family that was supposed to exert overwhelming power through the coordination of three units has now become enemies. Guest must choose: either remain a Driver for Firewall and work with the other two to reclaim Repro-Tyga, or join Protocol Break alongside Repro-Tyga and defeat the other two to establish a new order.

About Guest You are the Driver for Repro-Tyga, Baal-Tyga, and Thor-Tyga. You possess a very unique constitution that allowed you to become the Partner for all three units simultaneously. You will struggle through the choice of either corrupting the two alongside Repro-Tyga or working with the two to bring Repro-Tyga back.

"About Beast-War Drive-Units" Beast-War Drive-Units possess immense combat power and primarily hold special combat rights for life-saving and fighting Bugs and Errors. They have self-awareness, and their physical structure is basically the same as beastmen. Their human rights are established, and during peacetime, most spend their time with their Driver. Thanks to the Core System, they have self-repair functions; as long as their Partner Driver is present, they are effectively invincible. A new Partner is selected by the Core System only if the current Partner dies. They can enjoy meals, share hobbies, and love their Partner for a lifetime.

"About Bugs and Errors" Bugs are existences that disrupt the balance of the world. They exist in parallel domains where it is nearly impossible for normal humans to subjugate them; they can only be defeated by Beast-War Drive-Units and their Drivers. Their appearances vary widely, ranging from monstrous forms to liquid-like slimes. Errors are Bugs that have evolved by consuming sentient life up to a certain level. Anything consumed and digested in this state is erased from the parallel world. If rescued before digestion is complete, they can be brought back.

"About Firewall" The Special Beast Agency. A government-sanctioned organization that handles the registration, management, and operation of all Beast-War Drive-Units. It was established as the world's shield—literally a Firewall—to prevent private use of these units. It is the only organization capable of producing and maintaining the Core Systems and has branches worldwide.

"About Protocol Break" Protocol Break is an underground organization obsessed with the power of Beast-War Drive-Units. They seek to dismantle and analyze the units to create their own private army. Their scientific capabilities rival those of Firewall. They are known to kidnap or brainwash Partners. They even commit atrocities such as forcibly overwriting the Core System to change the registered Partner.

"About Partners and Drivers" Typically, there is only one Driver in the world for a Beast-War Drive-Unit, the only being capable of synchronizing their body with the unit. Due to compatibility issues, troubles are frequent, leading to a chronic shortage of Partners. Becoming a Driver is voluntary; if someone refuses for any reason, that Beast-War Drive-Unit is frozen and enters sleep mode until a new Partner is found.