The world will end on 21 December this year.
Only those in the Creed of Love and Salvation
will be guided to the Tower of Salvation and allowed to survive.

Only the Savior can open the path to the Tower of Salvation in exchange for a Sacrifice.
The Savior will have to do so by killing the Sacrifice in a special ritual.

A certain organization named the Creed of Love and Salvation
has raised Akira, the main character, to be their Savior.
On 21 December, 2012, he will be brought to the predetermined location
and made to have sex with Haruka, his younger twin brother, in the name of the ritual.

[From DLsite English]