Young-do whose father was a merciless murderer had to endure being called ‘monster’ ever since he was a child. His father’s dark shadow is constantly drawn over him despite his death by execution. Young-do vindictively lashes out to those calling him a monster, slowly becoming one himself. One day, a mysterious woman with a child shows up at Young-do’s house, claiming his father murdered her parents. When she demands for his heart, Young-do points a knife at his chest, daring her to stab him. He eventually fails to kick the woman and her child out of his house, and adding to this, he learns of a secret of hers. Shot in Busan, Shadow Island is the name of a location as well as the main character’s name. The film explains the legacy of violence passed down through three generations. The lives of a murderous father, a son addicted to violence, and the victims of violence intertwine into a surprising finale. As the film shifts through the past and present, it reveals a painful family history that eats away the soul and leads to a burdened fate of self-destruction.