To him, she was the world but to her, he was merely a voice. The moment he first saw Katerina, he knew he had to guide her and protect her. He was unperceived by all her senses, and he remains an unknown being only her mind could sense. Although he remained a spirit, he did his duties by watching over her and consoling her in times of trouble. However as time passed by, he found himself committing a huge mistake- falling in love with the girl who can't see him. Despite the circumstances, can Minhyun be her world and Katerina be his voice? Or will he remain a mystery with no leads for her to solve?