Chronos is an institution that guidelines one third of the world and covert business in assorted areas. Most of the Chronos member occupant intense positions, such as mayors, politicians, among others.
The Supreme Leader of Chronos decrees a secret force of the best killers in the world, vowed to defend Chronos, called the Chrono Numbers, which range from Number 1 to 12. Number 13 is being a special agent, and he is Train Heartnet, also known as Black Cat.