Love You All Kindergarten had to close, but its seven former students now attend Happy School, giving their parents some much-needed time off. As everyone adjusts, Class 1A's teacher, Miss Sin, surprises the parents by being a Happy School alumna who aims to restore its former glory. Strict and dedicated, she seeks to instill strong values and a solid educational foundation in her students, earning both respect and fear. However, the free-spirited former kindergarteners often push her limits, making them frequent targets of her scolding. Charles frequently clashes with Miss Sin over her strict methods but also supports her vision of nurturing elite students, leading to a mix of conflict and cooperation between them.