The Mickle Muckle Group decides to send 6 of its staff for retraining. The 6 staff members are: Fai, HR Cheung, Kau, Chun, Even, and On. Those who fail will be fired. The training location is an actual primary school with 6 actual Primary 4 students: Marble, Yau, Yu, Big Sis, Yuet, and Elly. Together, the 12 "classmates" form Class 4S. The class is led by Miss Yi, who is young, beautiful, and reserved. While encouraging and supporting one another, the adult students learn something new about themselves. With Marble's company, Fai begins to enjoy school. Strong bonds form among the classmates. However, as the end of the term nears, Fai breaks a school rule and is expelled from school. The training is also terminated. It turns out that the training scheme involved a bet between Kei, the principal of the school, and Wo, the owner of the Mickle Muckle Group.