Jung dae hyun is a Witness Protection Unit (WPU) officer whose team is assigned to protect the cellist Linda Kim, the daughter of wealthy businessman Kim dong ho, when linda witnesses the assassin who attempted to kill dong ho's brother Kim kyu ho.

Because she had seen the death of her brother at a young age, Linda has suffered from guilt and depression. Believing herself to be responsible for her brother's death, she gradually became anti-social and uses her music to isolate herself from others. dae hyun's appearance in her life causes linda to expand her horizons and she becomes happier. Despite her father's opposition, linda begins to pursue a romantic relationship with dae hyun.

As dae hyun works for the police, dong ho is hesitant to allow linda to continue her relationship with dae hyun because 's position could ruin dae hyun's status. To complicate matters further, his long-time enemy Paul son's, Leo reappears and threatens to unravel the origins of how dong ho obtained his wealth. When Dae hyun discovers the connection between dong ho and Leo's dad, he is torn between his duty to do what is just and his love for Linda.