Go Ae Rin is a single mother who after unexpectedly losing her husband, gets involved in a spy war. She lives in the house in front of Kim Bon. Kim Bon, who's 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. They both team up to uncover a huge conspiracy which Go Ae Rin's husband had become involved in. Joining them is Jin Young Tae, a former con man who may hold the key to what Go Ae Rin and Kim Bon are searching for, and Yoo Ji Yeon, a fellow NIS agent who apears cold and unfeeling on the outside but harbours feelings towards Kim Bon, who she is kind towards.