One day Subaru receives a weird book from an anonymous sender. The book has a putrid smell and is sealed. Within it is a message stating that she should never read it and that it’s impossible to dispose of, even by burning or cutting it to bits.
Rather than being disgusted or afraid, Subaru thinks of it as a good way to pass time and decides to show it off to Misuka and Kazumi. They decide to try burning it but not before Subaru reads a bit of it.

A few days later, strange things start to happen; the nice teacher who was Kazumi’s only ally disappears, strange people appear all around town looking for a book, and a transfer student immediately takes an interest in Subaru.

[Edited from hadlerblog]