Great proof of concept that works well enough despite its high complexity of plotline. The director had precise accuracy to execute on scenes that may seem impossible to achieve, yet providing a fresh new thrilling material for Korean action cinema. Its powerful soundtrack greatly heightens the suspense, coupled with incredible acting skills from Yoon Kye Sang that's very technical, making scenes look gripping and cinematic that you'll believe it's really happening in the way it's supposed to be.