"Himitsu Kessha Taka no Tsume Gaiden Mukashi no Yoshida-kun" is a spin-off of the popular "Himitsu Kessha Taka no Tsume" series, focusing on the backstory of Yoshida, a key character from the original franchise. The narrative delves into Yoshida's early life, exploring his transformation from an ordinary individual into a member of the secretive and eccentric organization known as Taka no Tsume. Set in a comedic and surreal tone, the series provides insight into Yoshida's motivations, quirks, and the events that shaped his personality.

The story begins with Yoshida as a young man, navigating the challenges of everyday life while harboring a deep fascination with conspiracy theories and secret societies. His life takes a dramatic turn when he encounters Taka no Tsume, a mysterious group dedicated to bizarre and often nonsensical missions. Initially skeptical, Yoshida is drawn into their world through a series of absurd and humorous events, eventually becoming a loyal member. The series highlights his interactions with other eccentric members of the organization, showcasing their peculiar dynamics and the group's unconventional approach to achieving their goals.

Throughout the series, Yoshida's character is fleshed out through flashbacks and episodic adventures, revealing his vulnerabilities, aspirations, and the reasons behind his unwavering dedication to Taka no Tsume. The narrative balances humor with moments of introspection, offering a deeper understanding of Yoshida's role within the group and his relationships with other members. The series maintains the absurdist humor and satirical tone of the original franchise while providing a fresh perspective on one of its most iconic characters.