"Unwritten" - An episode about Futaba and Kou in middle school. Based on the chapter called "Unwritten." also known as chapter 0 or Prologue. "Page.13" - An episode that continues the story after the television series's 12th and final episode. (Source: sakisaka10)

Imagine this: you are living a normal day in your life. Maybe you are out with friends, eating your family's home-cooked meal or spending time with your girlfriend. When you next wake up, you are suddenly thrust into a strange, new world, surrounded by five strangers on a rapidly sinking boat in the middle of a storm. For Natsu Iwashimizu, this is her new reality. Humanity has perished, and all that remains of the Japanese population are five groups of men and women who were chosen to be sent to the future in hopes of continuing mankind's existence. While every other person chosen has a useful talent such as martial arts, knowledge, or architecture, Natsu is a shy high school girl who cannot even raise her voice to shout. The new world is dangerous beyond imagination, and although Natsu seems to lack helpful skills, she must go with the others making their way to the "Seven Fuji" in order to survive. [Written by MAL Rewrite]

Aine Yuuki, who is a regular student at Star Harmony Academy's normal division, meets Mio Minato from the idol division, who invites her to join Aikatsu to fulfill Aine's goal to make friends. She also befriends Maika Chōno and Ema Hinata who are also idols. Aine and Mio form a pair to become best friends to become the bright "Diamond Friends". Karen Kamishiro and Mirai Asuku are categorized as "Diamond Friends" and are in the Diamond Class! (Source: Wikipedia)

The story takes place two years after Aikatsu Friends! with Aine Yuuki now in high school. One day she has an encounter with the mysterious idol Hibiki Tenshou who wears the shining Jeweling Dress.