"Eiga Oshiri Tantei: Tentō Mushi Iseki no Nazo" is a feature-length anime film based on the popular children's book series "Oshiri Tantei" (Butt Detective). The story centers on the titular character, Oshiri Tantei, a detective with a distinctive appearance—his head resembles a human buttocks. Alongside his assistant, Brown, a dog who often provides comic relief, Oshiri Tantei solves mysteries with a mix of humor and clever deduction.

The narrative begins when Oshiri Tantei and Brown are approached by a group of insects seeking help to solve a mystery involving an ancient ruin known as the Tentō Mushi Iseki (Ladybug Ruins). The insects explain that their sacred treasure, the Ladybug Jewel, has been stolen, and they suspect the culprit is a mysterious figure known as the Black Shadow. The theft threatens the balance of their ecosystem, and the insects fear the ruin's curse will bring disaster if the jewel is not returned.

Oshiri Tantei and Brown embark on an adventure to the Ladybug Ruins, encountering various challenges and puzzles along the way. The ruins are filled with traps and riddles designed to protect the treasure, requiring the detective to use his sharp intellect and unique abilities to navigate the dangers. As they delve deeper into the mystery, they uncover clues about the Black Shadow's identity and motives, leading to a confrontation that tests their resolve and teamwork.

Throughout the film, the story balances lighthearted humor with moments of suspense, appealing to its young audience while maintaining a sense of adventure. The characters, including the quirky Oshiri Tantei and the loyal Brown, are central to the narrative, with their interactions providing both comedic and emotional beats. The resolution of the mystery ties together the themes of friendship, perseverance, and the importance of protecting cultural heritage, delivering a satisfying conclusion to the adventure.