探偵団を結成したタカシたちにとって、当面の悩みは自分たちだけが使える秘密のアジトがないことだった。途方に暮れる５人だったが、諦めずに探していく中で、ついにうってつけの場所を見つけた。それは、なぜか河原の埋め立て地に埋まっていた、第２次世界大戦で使われたと思われるドイツのＵボート（潜水艦）だった。
After forming the Detective Group, Takashi and his friends are faced with the immediate problem of not having a secret hideout that only they can use. The five of them were at a loss, but they never gave up their search and finally found the perfect place. For some reason, they found a German U-Boat (submarine) buried in a landfill on a riverbank, which they thought had been used in World War II.
