"Deaimon" centers around Nagomu Irino, a young man who returns to his family’s traditional Japanese sweets shop, Ryokushou, in Kyoto after a decade-long absence. Nagomu had left home to pursue a career in music, but his dreams remain unfulfilled, and he comes back to a household that has changed significantly in his absence. His father, Kanau Irino, has retired, and the shop is now managed by Itsuka Yukihira, a mysterious and talented young girl who was taken in by the family years ago. Itsuka is deeply devoted to the shop and its traditions, and her presence complicates Nagomu’s reintegration into the family business.

The narrative explores the dynamics between Nagumu and Itsuka as they navigate their roles within the shop and their evolving relationship. Itsuka, who initially resents Nagomu for abandoning his responsibilities, gradually begins to see him as a mentor and ally. Nagomu, in turn, struggles to reconcile his past failures with his desire to contribute meaningfully to the family legacy. The story delves into themes of family, tradition, and personal growth, with the backdrop of Kyoto’s cultural and culinary heritage playing a significant role.

Supporting characters include Kanau, whose gruff exterior hides a deep affection for his family, and various townspeople who frequent the shop, each adding depth to the story through their interactions with Nagomu and Itsuka. The manga balances heartfelt moments with lighthearted humor, often highlighting the intricacies of running a traditional sweets shop and the challenges of preserving cultural practices in a modern world. Through its focus on relationships and the interplay between past and present, "Deaimon" offers a nuanced portrayal of life, responsibility, and the enduring value of family and tradition.