"Hyakuen!" is a slice-of-life manga that centers around the daily lives of two high school girls, Natsumi and Risa, who share a unique bond over their love for 100-yen shops. The story unfolds in a lighthearted and relatable manner, exploring their adventures as they navigate school life, friendships, and personal growth while indulging in their passion for affordable treasures.

Natsumi is a cheerful and energetic girl with a knack for finding joy in the simplest things. Her enthusiasm for 100-yen shops is infectious, and she often drags her more reserved friend, Risa, along on her shopping escapades. Risa, though initially hesitant, gradually warms up to the idea and begins to appreciate the charm of these budget-friendly stores. Together, they discover a world of creativity and resourcefulness, using their purchases to solve everyday problems, create DIY projects, and even inspire their classmates.

The narrative is episodic, with each chapter focusing on a new discovery or challenge tied to their 100-yen shop finds. From crafting handmade gifts to organizing school events on a tight budget, Natsumi and Risa demonstrate how small, thoughtful choices can lead to meaningful experiences. The manga also subtly touches on themes of frugality, sustainability, and the value of appreciating life's little pleasures.

Supporting characters, such as their classmates and family members, add depth to the story, often participating in or being influenced by Natsumi and Risa's 100-yen shop adventures. The interactions between characters are natural and grounded, reflecting the everyday dynamics of high school life. The manga's charm lies in its simplicity and relatability, offering a refreshing take on the slice-of-life genre by highlighting the joy of finding happiness in the ordinary.