After the spying mission of his father failed, North Korean boy Ri Myung-hoon and his sister Ri Hye-in are sent to a labor prison. Myung-hoon volunteers to become a spy to protect his sister and infiltrate South Korea as a juvenile detective. While at school as a regular student in South Korea, he met a girl also named Hye-in and rescues her when she is attacked. South Korean Intelligence detect Myung-hoons behavior and start tracing him while his government sent an assistant to assassinate him..