Du-su is a highschool student who is always smiling. Despite making many mistakes, he always seems to land on his feet. His warm heart and positive outlook on life endear him to all he encounters.

Nicknamed Yalkae, notorious high school prankster Du Su (Lee Seung Hyun) is always up to no good with his buddy Yong Ho (Jin Woo Young). Both of them have been held back a year, but instead of studying they continue to cause trouble for their teachers and classmates. Du Su gleefully pulls a prank on his tattling classmate Ho Cheol, but he has a change of heart when he finds out Ho Cheol gets injured as a result.