Go Ae-Rin (Jung In-Sun) suddenly loses her husband. A mysterious man, Kim Bon (So Ji-Sub), lives next door. Kim Bon is a legendary NIS agent. 3 years ago, he was involved in a failed secret operation and the woman he loved died. He has since become disconnected with the world and lives alone quietly. He helps his neighbor Go Ae-Rin uncover a conspiracy, which her husband became involved with.