Donghae is a professional assassin hired to take the lives of powerful people. Without any trace of conscience, without pity, he kills. He was not just an ordinary hitman though, he was the best of all of them - the king of the underworld, as they call him.
However, inside that cold aura, and disgustingly cruel line of work, he is a kind person - giving part of the money he earned to the orphanage where he came from. At that orphanage, he met Haeri.(Yuri), a volunteer worker for five months. She is a quiet but rather cheerful girl, who gave him hope for a better future. It was a comfortable friendship for him -he was always careful, hesitant in giving much information about himself, but she never demanded anything - like she understands his limit.
Somewhere along their quiet talks and playful daydreaming - he fell in love. It was ridiculous, the most stupid thing that an assassin can do.
It was a forbidden relationship that he tries so so hard to get out of, yet he can't.
He got the shock of his life when Ryeowook gave him his mission envelope, one day. It was his Yuri, only in a different name and background. Kwon Haeri-- daughter of the most powerful business tycoon in Seoul. He later found out that this is the hardest mission of his life -- killing the person he loved the most. So he asked Shindong (the boss of all bosses) to extend the duration of the mission, desperately planning how to get Haeri (Yuri) out of this mess, unscathed.
Another assassin, Eunhyuk found out about their relationship. Threatening Donghae that the organization would find out if he would not clear his mind and carry out with the mission. Donghae was determined to save Kwon Haeri (Yuri) though. Knowing this, the organization sends out Yesung and Siwon, two of the top assassins, to continue the mission Donghae failed to do.

(Note/: Please don't get confuse about Haeri and Yuri. They're just the same person, but then i made "Haeri" as Yuri's real name.)