"Mokkai Shiyo" is a comedic anime series centered around the daily lives of a group of high school students who form an unconventional club dedicated to creating and sharing absurd, often nonsensical jokes, or "mokkai." The narrative primarily revolves around the club's founder, Haruto, a quirky and eccentric student with an unrelenting passion for humor, even when his jokes fall flat. His best friend, Yuki, serves as the straight man, often exasperated by Haruto's antics but reluctantly participating in the club's activities. The group also includes Aoi, a shy but creative girl who secretly enjoys crafting elaborate pranks, and Riku, a laid-back transfer student who brings a unique perspective to their humor.

The series is episodic, with each episode focusing on the club's attempts to outdo each other in creating the most ridiculous or unexpected jokes. Their endeavors often lead to chaotic situations, both within the school and in their personal lives, as their humor inadvertently disrupts classes, confuses teachers, and bewilders their classmates. Despite the frequent misunderstandings and mishaps, the group's camaraderie grows stronger as they navigate the challenges of adolescence together.

A recurring subplot involves the club's rivalry with the school's more traditional comedy club, which views their approach to humor as immature and unrefined. This rivalry escalates into a series of comedic showdowns, culminating in a school-wide competition where both clubs must showcase their comedic talents. The series balances its humor with moments of heartfelt sincerity, particularly in exploring the characters' insecurities and aspirations, as they learn to embrace their individuality and find joy in their shared passion for laughter.