Luhan is the protagonist of this story. He is a famous high-school detective and the romantic interest of Seohyun. One day, he brings his childhood friend, Seohyun on a date and ends up solving a murder case there. He found two suspicious guys  and followed them secretly but is too engrossed to notice that there's another person behind him. He was knocked down and had been forced to swallow a new experimental poison known as APTX 4869. Instead of killing him, that poison shrinks him to six years old boy. To find more about the bad guys, Luhan changes his identity to Park Guntae and lives under the same roof with Seohyun and her father. As an elementary student, Park Guntae hangs out with other kids his age to cover his real identity. 

Problem arrived when Luhan's parents forced him to go to America with them and leave the black organization to the FBI. Luhan turned angry and said he has an extremely important reason why he has to stay in Korea. The presence of Sulli who faked her identity as Kim Soohyun drives Luhan and Seohyun apart.. especially when Luhan was ready to expose his real identity and confess his feelings to Seohyun. Despite Sulli being the one who create the drug that had shrunk him, Luhan was willing to sacrifice himself in order to save Sulli from danger. Sulli was heavily chased by Kris for unknown reason. But Luhan lowered his guard towards Kris after knowing that Kris is FBI's trumpt card.

A friend named Kai comes in need to help Luhan solved his problems. Unfortunately, they both becomes rival when it comes to investigation. Kai always wants to overcome Luhan's deduction skill but ends up in failure. Kai understands Luhan the most when Luhan is deadlocked. Besides being extremely good-looking and brilliant, the two of them have other things in common. Both of them are clueless about their own feelings and tends to prank someone they loved.

Another friend in need is Baekhyun. Baekhyun is actually Kaito Kid who often steals germs and then returns it back. Baekhyun accidentally heard Guntae conversation with Kim Soohyun which led him knowing about Guntae's real identity. Despite being enemy, Baekhyun always helps to cover Guntae's identity because he and Luhan are look alike. Without anyone knowing, Baekhyun is also involved with the black organization. He swears to crush the organization for killing his father who is the first generation of Kaito Kid.

Another person who knows about Guntae's real identity is Jessica who is a member of the black organization. Jessica first encountered Luhan and Seohyun in New York. She disguised as a serial killer and ready to kill Seohyun. But kind-hearted Seohyun saved her when she fell from the balcony. Seohyun was also dumbfounded why she bothered to save someone who wanted to kill her. Luhan drove all the curiosity away by stating that a logical mind is not needed to help someone, but a humanity is needed. From that incident, Jessica fell in love with Luhan and named him silver bullet.