Kasai Tadahisa, a handsome yet ordinary Japanese engineer who lived somewhere in Tokyo, died of overwork, and got reincarnated as a trashy third prince of Thaloria Kingdom, Third Prince Cain Vailtair Thaloria, in a mediavel world filled with knights and witches. However, he finds out he has a System that lets him access all modern blueprints from his previous life. He finds himself exiled by his father Alexander Cain Thaloria to the distant Frostmark Province, neglected by corrupt, negligent officials, and often overrun by wild shadow beasts and raiders from neighboring kingdoms. Will he take advantage of his System and turn the situation around?