A polite and meticulous sleuth with a face that resembles human buttocks, the investigator known as Butt Detective solves mysteries in a world of anthropomorphic animals. He wears a beret passed down from his father, Oshiri Dandy, and possesses an IQ of 1,104, a number that plays on the Japanese phrase for "good butt." His investigations typically begin with the catchphrase "Hmm, I smell a case" and conclude with him subduing criminals using his signature flatulence, often followed by "Pardon me while I poot you in your place." His loyal but frequently hapless partner is Brown, a small, earnest brown dog who often finds himself accidentally caught in the line of the detective's unique crime-fighting technique. The detective enjoys sweet potatoes and a good cup of tea after work, while his greatest weakness is spicy food.

The setting is a cheerful, storybook-style town where the local police, led by the tiny white dog Chief Maltese of the汪汪 Police Department, frequently seek the detective's assistance. The series follows a largely episodic format, with each installment presenting a new puzzle, from tracking a candy thief and investigating ghost sightings in a clock tower to recovering stolen treasures like the Rainbow Diamond. A recurring rival is the flamboyant phantom thief known as Phantom Thief U, a master of disguise with a distinctive aesthetic who enjoys issuing playful challenges before his heists. Despite his criminal nature, he adheres to a personal code of not harming others and often finds himself outsmarted by Butt Detective's unexpected methods.

While most episodes are self-contained, the series features longer narrative arcs, particularly in its theatrical films. In the film Butt Detective: Star and Moon, the detective is hired by a koala to find a missing person from an idol contest, a case that leads him to Stardust Island. There, he confronts the Phantom Thief Academy, an international criminal organization led by the villainous Phantom Thief G, who is executing a secret plan known as the Dark Age Plan. To succeed, Butt Detective goes undercover as an idol and forms a tentative, pragmatic alliance with his old rival, Phantom Thief U, to retrieve the stolen Moonlight Stone. This story also explores previously unrevealed aspects of Brown's past.