"Happy Lesson" centers around Chitose Hitotose, a high school student who lives alone after the death of his parents. His solitary life takes an unexpected turn when five of his teachers move into his home, each assuming the role of a mother figure. The teachers—Minazuki Rokusue, Yukino Houzuki, Kisaragi Satsuki, Uzuki Hazuki, and Kisaragi Mutsumi—represent different personalities and teaching styles, creating a dynamic and often chaotic household. Each teacher brings her own quirks and challenges, from Rokusue’s strict discipline to Satsuki’s carefree and mischievous nature.

The series unfolds as a slice-of-life comedy, with episodic stories focusing on the interactions between Chitose and his new "mothers." The teachers’ attempts to care for him often lead to humorous misunderstandings and heartfelt moments, blending comedy with light drama. As the series progresses, the relationships deepen, revealing more about the teachers’ backgrounds and their reasons for taking on this unconventional arrangement. Themes of family, responsibility, and personal growth are explored through their shared experiences.

Chitose, initially resistant to the idea of having five mother figures, gradually warms up to their presence, finding a sense of belonging he had long been missing. The teachers, in turn, learn from Chitose and each other, forming a unique family unit. The narrative balances comedic situations with emotional undertones, creating a lighthearted yet meaningful exploration of unconventional family dynamics. The series concludes with a sense of closure, as the characters reflect on their journey together and the bonds they have formed.