From the game introduction:
Have you ever seen one of those movies where a team of bold explorers faces a threat invisible and deadly, and they manage to get rid of it only after long minutes of discomfort and sheer terror, when almost all its members have been killed or gone mad? You've probably played also this team, am I wrong? How about this time you become the threat, the monster? Would you accept the task to coldly eliminate these invaders, to terrorize and kill anyone that threatens your territory, your mission? That's what Larmes de Rouille (Tears of Rust) offers you... Become the gatekeepers of Rust, ultra-powerful, invincible, immortal, implacable, without the slightest trace of emotion... until an emotion is born in you! Then, perhaps, it will be your turn to know fear...