Train Heartnet, known as the infamous gunman Black Cat, is a cold‑blooded assassin who serves the organization Chronos and bears the mark XIII on his body. Raised amid violence, Train has become quiet, asocial, and indifferent to others. His life takes a turn when he meets the enigmatic Sweeper, Saya, whose unconventional way of living captivates him. A subsequent tragedy forces Train to abandon his old path, and he embraces a new life as a Sweeper, forging bonds with new friends Sven and Eve.