Main Stories are the main events in A3!. They are separated by Acts, and in Acts, there are Episodes with multiple Chapters. Main Story can be unlocked by earning EXP to ranking up or during Campaigns. You get 2 Gems after reading a story chapter.
Act 1 Episode 1 - 4 can be unlocked by ranking up, while Act 2 Episode 5 - 8 can be unlocked after Rank 55 and by gaining an specific amount of EXP.