Kang Soo is a loan shark, who just can't catch a break. After another hard day of nothing but disappointment, Kang Soo finds a poster asking if he wants to become happy. Sick of the life he's been living, he wants a change. In hopes of experiencing happiness for the first and possibly last time in his life, he goes to the research center on the poster.

After finding out that participation in the research will cost him 300,000 dollars, he goes to the one place he knows where such money exists. He manages to get the money and goes back to the research center. To his dismay, it is no longer there. He brings the money home, to contemplate his next step. Meanwhile, Hwang Tae Bok goes after Kang Soo to get his money back.

After Kang Soo’s motorcycle accident, Jeong Man is told to prepare for the worst. Jae Hui approaches Jeong Man and offers the chance for Kang Soo to participate in the happiness research. Jeong Man agrees because he believes it's the only way to save Kang Soo. Meanwhile, Kang Soo begins the experiment, but amidst the happiness, he begins to believe that something isn't right.

Once the seed of suspicion has been planted, Kang Soo begins to question everything that has been going on. A strange man keeps telling him to get out. Every time Kang Soo tries to express his concerns with various people, he is met with the same answer. Things begin to fall apart, and he doesn't know why. Meanwhile, he wakes up the next day, and everything is back to perfect.