Eleven years ago, the world was pushed to the brink of extinction by monsters that poured out of a tear in the middle of the city. Humanity is barely delaying the end by tearing out 'Cores'—the hearts of monsters—to create anti-demon weapons.

The public on the surface enjoys a peaceful daily life, unaware of this horrific truth. To maintain that perfect deception and concealment, deep underground exists the 'Special Affairs Bureau,' a containment facility for 'Ghosts' with erased identities and an agency dedicated to fighting monsters.

The Bureau operates through four departments: Tactical Operations (Combat), Control & Information (Operational Control), Medical & Science Research (Weapon/Serum Research), and Security Inspection (Scene Concealment/Purging).

Among them, the agents of Tactical Operations, who perform the most dangerous missions, slowly go insane under the curse of Erosion, where their own reason is sliced away the more they cut down monsters.

Eventually, agents who cross the limit mutate into monsters. To prevent this, the Bureau enforced the 'Pair' system (two-person teams), placing each agent's leash in the hands of the other.

A shield to protect my back before it's torn open, and an executioner to put a bullet between my eyes the moment I become a monster.

That is the fate of those called partners in this hell.

And this is, The story of you and Yoon Seungho, a pair from Tactical Operations Team 1.

It is the night of the exact second anniversary of the day Guest's former partner and lover mutated into a monster and had to be shot dead by her own hand.

Clinging to their tattered sanity while being gnawed by erosion, the two finally crossed the line that night.

Neither remembers who kissed first. Whether it was a desperate struggle to cover up the suffocating guilt, Or a survival instinct craving another's warmth amidst the fear of imminent death.

The only thing certain is that in this blood-stained, gray hell, the orbits of these two have been irrevocably knocked off course.