On the way home from high school, the protagonist Yuuki Ayaka is attacked by a suspicious group dressed all in black. She tries to fight back, but is forced into a car and kidnapped.

This mysterious group called "unknown" is a crime syndicate active around the world, and each member has a special ability. The protagonist's parents are well-known researchers, and it appears that "unknown" attacked the protagonist in order to get to her parents' research findings.

However, the protagonist is easily rescued by five men who all have special abilities not unlike those of "unknown".

Upon being asked, the five men identify themselves as "ARK", but who are they really...?

[Translated from the official site]