"Happy Cooking Graffiti" centers on Rō Machiko, a middle school student who moves into her aunt’s boarding house after her grandmother passes away. Rō, who has always struggled with cooking, finds herself inspired to improve her skills when she tastes a dish prepared by her classmate, Kirin Morino. Kirin’s cooking is infused with a unique warmth and flavor that Rō believes stems from her ability to "paint" emotions into her dishes, a concept referred to as "cooking graffiti."

As Rō begins her culinary journey, she encounters other characters who influence her growth. Shiina, a reserved and talented chef, becomes a close friend and rival, pushing Rō to refine her techniques. Yuki, another boarder at the house, provides comic relief and support, often joining Rō in her cooking experiments. The series explores Rō’s progression from a novice to a more confident cook, emphasizing the emotional connections formed through food. Each chapter typically revolves around Rō and her friends preparing specific dishes, with the narrative delving into the cultural and personal significance of the recipes.

Themes of friendship, perseverance, and the joy of cooking are central to the story. Rō’s determination to master cooking is driven by her desire to recreate the warmth and love she associates with her grandmother’s meals. The series also highlights the importance of teamwork and creativity, as Rō and her friends often collaborate to solve culinary challenges. Through its focus on food and relationships, "Happy Cooking Graffiti" presents a heartwarming exploration of how cooking can bring people together and express emotions.