Forty-year-old detective Eiji Kirisame encounters an unusual witness at a crime scene: Tsugumi Kuchinashi, a high school girl who never speaks. She communicates exclusively through crude but insightful doodles on her sketchbook. Despite her silence, Eiji recognizes a sharp deductive mind behind the drawings and takes her scribbled statements seriously as he pieces together the truth from limited evidence. The strangest part of her testimony arrives on the final page of her sketchpad: Will you marry me?

This question sets the tone for an unconventional partnership that blends mystery, comedy, and romance. The setting is a contemporary Japanese city where Eiji works as a serious, work-focused detective. Tsugumi becomes his unofficial partner in solving criminal cases, using her artistic abilities to visualize clues and reconstruct crime scenes that his colleagues fail to notice. The narrative follows a recurring pattern: the duo encounters a suspicious incident, Tsugumi produces revealing sketches that others dismiss as childish, and after a back-and-forth of deduction, the truth emerges in a gotcha style reminiscent of classic mystery formulas.

The central dynamic hinges on the 22 year age gap between the protagonists and their contrasting positions in society. Eiji is a seasoned professional who respects evidence and procedure. Tsugumi is a silent outsider whose only voice comes through her sketchbook. Her persistent marriage proposal hangs over every investigation, transforming routine police work into an awkward romantic comedy. Eiji must balance his professional responsibilities with the bizarre personal situation of being courted by a teenage witness who refuses to take no for an answer.

Notable narrative arcs focus on specific criminal cases that challenge the duo in different ways. Each mystery follows a similar structure: the pair is spending time together, a crime or suspicious event occurs, Tsugumi lights up with inspiration, and her drawings provide the key to unraveling the villains elaborate schemes. The series heavily relies on the conceit that most other characters, including fellow police officers, are incompetent compared to the mute girl's deductive abilities. Some cases explore why Tsugumi chooses to communicate through doodles rather than simply writing out the culprit's name, a quirk that becomes part of the series comedic identity.

The creators have noted the inherent challenge of adapting a story where the heroine speaks no dialogue. The anime adaptation preserves this silent protagonist conceit, relying on visual storytelling, background music, and the reactions of other characters to convey Tsugumis thoughts and emotions. The series balances lighthearted comedic interactions between the leads with the procedural tension of solving murders and other serious crimes.