Long before the famous Butt Detective began solving cases, his father, Oshiri Dandy, was a young adventurer working as a ruins investigator at the Museum Beyond the Sea. The story follows Dandy and his knowledgeable but cautious assistant, Hehe, as they travel the globe on a mission to protect the world's treasures from those who would steal them.

The television series unfolds through a dual narrative structure. In the primary storyline, the young Oshiri Dandy embarks on daring expeditions to ancient sites like Monkey Shrine in search of legendary artifacts. These adventures are full of peril, from navigating treacherous river rapids on fragile rafts to outsmarting complex booby traps and competing with rival treasure hunters, such as the遗迹小偷阿拉丁. Dandy approaches every challenge with a confident and charming demeanor, living by his motto, "This is the behavior of a gentleman," and showing a particular weakness for helping women in need. His favorite snacks are sweet potato chips and sugary canned coffee.

Interwoven with these flashback adventures is a second storyline set in the present day. The adult Butt Detective investigates a mysterious message sent to him by the phantom thief Kaito U, which leads him into a direct confrontation with the Phantom Thief Academy, a rival organization. This parallel plot allows the series to explore the legacy of adventure and deduction passed down from father to son while maintaining the classic mystery-solving elements the franchise is known for.

The young Oshiri Dandy is portrayed as a帅气和 capable冒险家 who works to preserve historical ruins, while his partner Hehe is a bookish history enthusiast whose intelligence is often hindered by his timid nature. The series weaves together action, comedy, and mystery, with each episode presenting new puzzles and obstacles that Dandy and Hehe must overcome using their wits and bravery. The anime began airing as part of the main Butt Detective television series starting April 4, 2026.