The 2nd Chapter continues the story of Ryo Takatsuki and his companions' quest to discover the secret of the mysterious A.R.M.S. they all possess and what the future may hold for them. They now know where the answers lie but to get to them, they must fight a desperate fight of survival against a powerful military corporation that wants their secrets no matter what the cost. (Source: AniDB)

Satake Hirofumi is a prisoner on death row for murdering his fiancée. He's also an "Undeadman" who has survived many desperate situations. For this, he has earned the interest of a research institution called "Human Crazy University," which studies real-life miraculous phenomena and the people who become entangled in them. Now a subject of their research, Satake relates to them his memories of his immortal, yet unhappy, life. Why did he kill his fiancée? Soon enough, the truth sheds light on a much bigger conspiracy...

It is the dark century and the people are suffering under the rule of the devil, Vandel, who is able to manipulate monsters. The Vandel Busters are a group of people who hunt these devils, and among them, the Zenon Squad is known to be the strongest busters on the continent. A young boy, Beet, dreams of joining the Zenon Squad. However, one day, as a result of Beet's fault, the Zenon squad was defeated by the devil, Beltose. The five dying busters sacrificed their life power into their five weapons, Saiga. After giving their weapons to Beet, they passed away. Years have passed since then and the young Vandel Buster, Beet, begins his adventure to carry out the Zenon Squad's will to put an end to the dark century.

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)