"Lunny-chan to Tanjōbiya-san" centers around the whimsical and heartwarming relationship between Lunny-chan, a cheerful and curious young girl, and Tanjōbiya-san, a mysterious figure who appears on her birthday. The story unfolds in a quaint, fantastical town where birthdays are celebrated with great significance, and Tanjōbiya-san is known as the "Birthday Bringer," a magical being who ensures that each child’s birthday is special. Lunny-chan, however, is unique—she has never had a proper birthday celebration due to her family’s circumstances. When Tanjōbiya-san arrives on her doorstep, he offers to make her birthday unforgettable, setting off a series of magical adventures.

The narrative explores their journey as they travel through enchanting landscapes, encountering various challenges and whimsical characters. Each episode introduces a new aspect of the world, often tied to themes of friendship, self-discovery, and the importance of celebrating life’s milestones. Lunny-chan’s boundless optimism and Tanjōbiya-san’s enigmatic yet caring nature create a dynamic that drives the story forward. As they work together to overcome obstacles, Lunny-chan learns more about herself and the world around her, while Tanjōbiya-san gradually reveals his own backstory and the reasons behind his role as the Birthday Bringer.

The series balances lighthearted moments with emotional depth, particularly in its exploration of Lunny-chan’s longing for connection and Tanjōbiya-san’s sense of duty. Supporting characters, such as Lunny-chan’s neighbors and other magical beings, add layers to the narrative, enriching the world and providing additional perspectives on the central themes. The story culminates in a heartfelt resolution that ties together the threads of Lunny-chan’s personal growth and the magical lore of the Birthday Bringer, leaving a lasting impression of warmth and wonder.