"Ojisan to Marshmallow" is a comedic slice-of-life manga that explores the quirky dynamics between its two main characters, Habahiro Hige, a middle-aged office worker, and Wakabayashi, a young woman with an unusual obsession. The story revolves around their interactions, which are often centered on marshmallows, a food item that serves as a recurring motif throughout the series. Habahiro, despite his gruff and reserved demeanor, has a secret love for marshmallows, a fact that Wakabayashi exploits to tease and manipulate him in various humorous situations.

Wakabayashi, who works in the same office as Habahiro, is playful and mischievous, often using marshmallows as a tool to get closer to him. Her antics range from offering him marshmallows in creative ways to staging elaborate scenarios that force him to confront his affection for the treat. Despite his initial resistance, Habahiro gradually becomes more accepting of Wakabayashi's presence and her marshmallow-themed schemes, leading to a series of lighthearted and often absurd encounters.

The manga is episodic in nature, with each chapter presenting a self-contained story that builds on the growing rapport between the two characters. Their relationship evolves from one of mild annoyance and awkwardness to a more comfortable and mutually understanding dynamic, though it remains firmly rooted in comedy. The humor arises from the contrast between Habahiro's stoic personality and Wakabayashi's relentless enthusiasm, as well as the absurdity of their marshmallow-centric interactions.

Supporting characters occasionally appear to add variety to the narrative, but the focus remains primarily on Habahiro and Wakabayashi. The series does not delve deeply into dramatic or emotional arcs, instead maintaining a consistent tone of light-hearted comedy. The simplicity of the premise and the repetitive yet inventive use of marshmallows as a narrative device contribute to the manga's charm and appeal. Overall, "Ojisan to Marshmallow" offers a straightforward, humorous exploration of an unconventional relationship, driven by its unique central theme and the chemistry between its two leads.