To fairly summarize, Moto, somebody who scavenges stuff to sell, meets with Moto after a fierce fight and they become good friends going on missions together. But after encountering a demon and fusing together, they work for Moto's brother, Nate. But in the shadows, Notokawi was slowly putting in his plan for the fall of the N company (or Nate's company, not in the book but we found out through other means) and he did this through making Moto angrier to hurt his friend and also expose Nate for his exploitation. They defeat him and the entire existence blows up into infinite alternate universes and now they're separated.
It's a pretty straightforward story, not digging into the worldbuilding a lot, just a story about how the great expansion happened