Toaru Majutsu no Index: New Testament - Volume 12, Chapter 1: Steamed Buns and Diamonds — to_the_DIANOID.

The world experienced a cataclysmic change, and countless spatial gateways appeared out of thin air.

The story is about a boy entering high school, who answers a strange call from a public phone one night. The caller challenges him to a bet: She’ll remove the birthmark which he sees as the cause of so many problems, and he’ll have fifty days to win the heart of a girl he’d considered far beyond his reach. And with this, a very strange summer begins.

In a magical medieval world, it's difficult to hold on to one's beliefs... even maintaining one's integrity is a challenge.Because the commoners here are not simple; they are ignorant, greedy, and cruel.Because the nobles here are not noble; they are insidious, ruthless, and tyrannical.However... not all human constitutions can be generalized.Lann touches the two hearts and three lungs beneath his chest.He estimates that not only in this world but also in the other worlds connected through the convergence of celestial spheres, he can maintain a healthy sense of morality.—And then get rid of all the incorrigible scum!

High-school student Koremitsu Akagi finds himself attending the funeral of Hikaru Mikado, who had died in an accident during Golden Week. He’s not sure why he’s there, as he only exchanged a few words with Hikaru at school. However, Hikaru doesn’t seem to be completely gone from the world, as he comes back as a ghost in order to fulfill a promise to Aoi Saotome, his fiancée. Since Koremitsu is the only one who can see him, he asks him to carry out his promise in his place.

Complying with his request, Koremitsu tries to fulfill Hikaru’s promises to the rest of his “girlfriends”, helping them overcome past misunderstandings and repair broken relationships.