"Eiga Oshiri Tantei: Star and Moon" is a feature-length anime film based on the popular children's book series "Oshiri Tantei" (Butt Detective). The story revolves around the titular character, Oshiri Tantei, a detective with a distinctive appearance—his head resembles a human buttocks. Alongside his assistant, Brown, a dog with a penchant for solving puzzles, Oshiri Tantei tackles mysteries with a blend of humor and deductive reasoning.

The narrative begins with Oshiri Tantei and Brown receiving a mysterious invitation to a grand party hosted by the enigmatic Star and Moon. The invitation leads them to a luxurious mansion where they encounter a series of peculiar guests, each with their own secrets. The central mystery unfolds as valuable treasures begin to disappear during the party, and the guests start pointing fingers at one another. Oshiri Tantei takes it upon himself to uncover the truth behind the thefts, using his keen observational skills and logical thinking to piece together the clues.

As the investigation progresses, the story introduces a cast of quirky characters, including the flamboyant host Star, the reserved and mysterious Moon, and a variety of eccentric partygoers. Each character adds layers to the mystery, with their motivations and actions becoming key to solving the case. The film maintains a lighthearted tone, balancing its detective elements with comedic moments and playful interactions between the characters.

The plot thickens as Oshiri Tantei discovers that the thefts are part of a larger scheme orchestrated by a cunning mastermind. The detective must navigate through a series of clever traps and red herrings, all while ensuring the safety of the guests and recovering the stolen treasures. The climax features a dramatic confrontation where Oshiri Tantei reveals the true culprit, tying together the various threads of the mystery in a satisfying resolution.

Throughout the film, the dynamic between Oshiri Tantei and Brown provides both comedic relief and a sense of camaraderie, highlighting their teamwork and mutual respect. The story emphasizes themes of friendship, perseverance, and the importance of looking beyond appearances to uncover the truth. "Eiga Oshiri Tantei: Star and Moon" delivers an engaging and entertaining mystery suitable for younger audiences, with a narrative that keeps viewers guessing until the very end.