Tantei wa Mou, Shindeiru.

Kimihiko Kimizuka has a tendency for getting involved in trouble, and one day, he's kidnapped and forced onto a hijacked flight. But then he meets a beautiful silver-haired woman who calls herself Siesta, and she saves the day. She claims to be a legendary detective, and insists that Kimizuka become her sidekick. After a lot of convincing, Kimizuka agrees and the two embark on a grand adventure across the world, preventing threats that could spell disaster for humanity.

But then, after three years of travel and excitement, Siesta abruptly passes away. Kimizuka is heartbroken, and tries to put all memories of her behind him. But as he meets more people, it's clear that her influence will never truly die.