On an ordinary night, Myoyou leaves home to buy something from a convenience store, only to witness something impossible.

A mysterious portal appears in the middle of the street, and from it emerges a severely injured man named Kai. Claiming to be from another universe but suffering from amnesia, Kai remembers only that the portal is connected to time travel and alternate universes, and that something has gone terribly wrong.

After tragedy strikes, Myoyou convinces Kai to travel back in time to change the past. They succeed, but reality begins to unravel. Memories no longer match, people behave in impossible ways, and someone appears claiming to have known Myoyou his entire life.

Believing the answers lie beyond their own universe, the two step into the portal and begin a journey across parallel worlds to uncover the truth behind the portals, the shifting timelines, and Kai's forgotten past.