Approximately 7 years after the end of World War I. On June 2, 1925, as the League of Nations was established and the world was about to take a step toward peace, at 4:17 AM, unprecedented spatial anomaly phenomena occurred simultaneously in various parts of the world. This phenomenon would later be called the "Zero Break." The phenomenon was confirmed in over a dozen locations worldwide, including Tokyo in Japan, the outskirts of Berlin in Germany, and the London Underground in the UK. The damage was very limited, but reports of inexplicable events followed:

- Building walls vanishing for just a few seconds
- Humans appearing to teleport several meters away
- Non-existent streetscapes appearing for dozens of seconds
- Discovery of unknown minerals and plants

However, scientific technology was underdeveloped at the time, making it impossible to investigate the cause. Governments suppressed the information, and many documents were classified. Decades later, it faded from memory as a "post-war mass hallucination," "tabloid sensationalism," or "urban legend." In reality, however, this was the day the Layer, the boundary separating our world from another, first cracked.

90 years later, in 2015. The weak Zero Breaks observed worldwide reached their limit. At 11:32 AM, above Tokyo, a massive spatial rupture occurred. A giant glass-like crack ran through the sky, and from within it, another Tokyo appeared. It was a Tokyo that existed in a parallel world different from Earth. A few seconds later, the two Tokyos completely overlapped, and buildings, roads, subways, humans, and space itself merged. The greatest paranormal disaster in world history occurred. The damage was catastrophic:

Deaths: Approx. 9,215,472
Missing: Approx. 5,083,886
Seriously Injured: 2,628
Destroyed Buildings: Approx. 6,920,000
Evacuees: Approx. 1,080,000

The Japanese government declared a state of emergency. Although the Self-Defense Forces, police, and fire departments were fully mobilized, damage from unknown entities expanded. In light of the damage, the entire Shibuya Ward was sealed off (as of 2034, it remains a Class 1 Exclusion Zone, prohibited to the general public). Furthermore, starting from a certain period, multi-dimensional lifeforms called Rentra—born from the forced fusion of laws and energy from the other world with objects and energy of the real world—appeared and began to run rampant. Taking these incidents seriously, the government named the series of phenomena Layer Crack and established the Otherworld Defense Agency, a global defense organization directly under the Cabinet Secretariat.