Sickness is a Visual Novel following the life of Suoh, a high school student turned violent criminal.
Following the deaths of his parents, Suoh is left without a home or source of income. All he has is his twin sister, Sara, who he promises to provide for, even if doing so means throwing his life away.
Using the entirety of their savings, the two move to another city, wherein Suoh quickly finds work. Sara commutes to her old school, Suoh works long, physically-demanding hours, and the two survive off instant food, barely scraping by. It isn’t a happy life, but they make it work.
Or at least they did, until a violent outburst set into motion a rapid descent.
Amid a physical altercation with his boss, Suoh blacks out, waking up to find a corpse laying at his feet. His knuckles bloody, his muscles aching, he realizes that he has just murdered his boss, the person responsible for his continued well-being. But rather than lament the loss of life, all Suoh can think of is how badly he needed the money, and what he’ll do next to get it.
Drawn by the scent of blood, one of Suoh's coworkers, a young man with criminal connections, offers him a not so legal revenue stream, one encouraging violent behavior and rewarding the capable.
And from there, everything spirals out of control.
As it turns out, Suoh has a knack for crime, and the petty reservations he had about such acts don't last long. For the well-being of his sister, as well as his own self-interest, he becomes a killer for hire, taking out people anybody his new boss tells him to. He quickly adapts to life as a killer, adopting a double life his sister can never know about.
But his new life is not without trouble. Facing death on a weekly basis is only the beginning; between avoiding police detection, eyewitnesses, and even the malice of his fellow professional killers, Suoh's new life looks to be the death of him.
Will Suoh manage to survive long enough to keep his promise to his sister? Will he turn tail and escape from this life of crime, finding solace in the company of someone or something else? Or will he be hunted down and killed before he ever gets the chance?