Haegu, a proficient and merciless hitwoman, executes a man in front of his own daughter and takes the child under her wing. She decides to name her Mian, the word for "sorry" in Korean, and gives her one mission: "Kill me when you can." From that moment on, Mian decides that she will do this by choking Haegu with feigned love and emotionally manipulating her into submission.

But as Mian grows into an adult, she's thrown into confusion as what was supposed to be pure revenge turns into lust... and possibly real love. Meanwhile, Haegu grapples with her own feelings for Mian while continuing to carry out life-threatening missions. Can Haegu and Mian ever learn to truly love each other, or are they doomed to drown in their toxic cycle of violence and revenge?