"Eiga Oshiri Tantei Shiriarty" is a feature-length anime adaptation of the popular children's book series "Oshiri Tantei" (Butt Detective). The story centers on Shiriarty, a brilliant detective with a distinctive appearance—his head resembles a human buttocks. Despite his unconventional look, Shiriarty is highly skilled in solving mysteries, often accompanied by his loyal assistant, Brown, a dog with a penchant for sweets.

The narrative unfolds as Shiriarty and Brown are called upon to investigate a series of peculiar crimes orchestrated by the enigmatic villain, Dark Lord. Dark Lord, a master of disguise and deception, seeks to spread chaos and confusion, challenging Shiriarty's deductive abilities at every turn. The detective's investigations take him through a variety of whimsical and imaginative settings, each presenting unique puzzles and obstacles that require his sharp intellect and resourcefulness to overcome.

Throughout the film, Shiriarty employs a combination of logic, observation, and humor to unravel the mysteries. His methods often involve playful interactions with the suspects and witnesses, blending lighthearted comedy with the thrill of solving crimes. The dynamic between Shiriarty and Brown adds a layer of charm to the story, as their partnership highlights teamwork and mutual support.

The plot is structured around a series of interconnected cases, each contributing to the overarching conflict with Dark Lord. As Shiriarty delves deeper into the mysteries, he uncovers clues that gradually reveal Dark Lord's true motives and identity. The climax of the film features a high-stakes confrontation between Shiriarty and Dark Lord, testing the detective's resolve and ingenuity in a battle of wits.

The film maintains a family-friendly tone, with its humor and storytelling tailored to engage younger audiences while offering enough intrigue to captivate viewers of all ages. The characters are designed to be memorable and endearing, with Shiriarty's unique appearance and quirky personality standing out as a defining feature of the series. The narrative emphasizes themes of perseverance, problem-solving, and the importance of curiosity, making it both entertaining and educational for its target audience.