They are agents sent by TOA Heavy Industries to retrieve humans with the ability to resist and transmute the N5S infection originating from Mars, which is spreading across the world fast, turning humans into "Drones"; disfigured, zombie-like beings.

They are the creators of synthetic humans and have made it their priority to collect those who have adapted to the N5S virus before the DRF does.

On February 26, 3006 A.D. TOA Heavy Industries's headquarters self-destructs, leaving the synthetic humans on their own.

The immortals are humans who have adapted to the N5S virus on a genetic level (having 24 pairs of chromosomes after infection), integrating it into their cell structure and giving them inhuman abilities: advanced regeneration and longevity.

[b] Shueisha collected its chapters in six tankōbon volumes, released from January 19, 2007,[9] to March 19, 2009.