Go Ae Rin is a single mother who, after unexpectedly losing her husband, becomes embroiled in a spy war. She lives in the flat opposite Kim Bon. Kim Bon, whose codename is Terius, is a legendary covert operations agent with the National Intelligence Service (NIS) who went into hiding after disappearing without a trace. Three years ago, during a failed covert operation, he lost the woman he loved. Since then, Kim Bon has been hiding in his flat, next to Go Ae Rin’s, isolating himself from the world around him and living in secret in an attempt to unravel the mystery behind what happened back then. When a mysterious incident occurs in which Go Ae Rin’s husband dies, the two join forces to uncover a massive conspiracy in which Go Ae Rin’s husband was involved. They are joined by Jin Young Tae, a former con artist who may hold the key to what Go Ae Rin and Kim Bon are searching for, and Yoo Ji Yeon, an NIS agent who appears cold and unfeeling on the surface, but who harbours feelings for Kim Bon, towards whom she is kind.