Plot: Go Ae Rin is a single mother who, after unexpectedly losing her husband, gets involved in a spy war. She lives in the apartment in front of Kim Bon. Kim Bon, who is code name is Terius, is a legendary black ops agent for the National Intelligence Service (NIS) who has gone into hiding after disappearing without a trace. 3 years ago, in a failed secret operation, he lost the woman he loved. Since then, Kim Bon has hid away in his apartment, next to Go Ae Rin, isolated from the world around him and living in secrecy trying to uncover the mystery behind what happened back then.