Two years ago, six school chums - Ta, Kim, Por, Tair, Noot, and Ko - are faced with a grudge. Ta's mother died when he was young, and his father remarried Ta's teacher, Miss Panor. During their last year at school, Miss Panor seduces Por. Por subsequently discovers he's not the only one in her bed. In addition to being married to Ta's father, Miss Panor is also having an affair with the sports coach. Fuelled by jealousy, Por suggests filming Panor and the coach to 'prove her infidelity' to Ta's father. The friends, save for Kim, do so, and broadcast it to the entire school. The coach soon discovers it was them and holds the group at gunpoint while he sexually abuses them. Seeking revenge, the students approach a monk who agrees to curse the coach. A few days later, Por goes back to the monk and asks him to curse Miss Panor, too. The coach dies in which numerous fish hooks appear from his body. Miss Panor, who is embarrassed at her sexual exploits being revealed, is found stabbing herself repeatedly in the legs. Years later the group reunites to pay respects to the teacher because her husband has passed away. Little do they know what horrors await them.
It's 2016 and a wedding is being held. A group of friends are reunited on this occasion nearly 20 years after their separation. Flashback to May 5, 1995, when they were still in high school. A Good Day tells the story of the youth and struggle of a group of friends that ultimately shapes their present adult lives. Can they find peace with their past and each other to reach a better tomorrow?