Mattaku Saikin no Tantei to Kitara

Keiichirou Nagumo was once a brilliant teenage detective, but time has not been kind to his reputation. At thirty-five, he is broke, hopelessly behind on modern technology, and struggling with cases that should have been simple. Then Mashiro, an energetic high school girl, appears and insists on becoming his assistant. Her familiarity with the modern world and relentless enthusiasm collide with Nagumo's old-fashioned experience and stubborn pride, creating an unlikely detective team that somehow keeps finding answers while each tries to prove that their way of solving a case is the better one.