Tantei wa Mou, Shindeiru. The Detective is Already Dead
Double-length first episode
During an airplane hijacking, a boy is suddenly pulled into becoming a girl detective’s assistant in order to resolve the incident. For the next couple years, he experiences adventures as the detective pursues a mysterious organization. However, this is all in the past. After the detective’s death, he is approached by a strange girl, who seems weirdly familiar, with a mysterious request. The detective is already dead, but something about her--and her mission--lives on.
[Source: Wakanim Nordic]