Rough summary-
The kid killed his family and ran away. He bumped into the detective and threatened him to make him come along. They backpacked around, and then the detective found out that the kid was a murderer. He told the kid to turn himself in. They had sex and then the kid killed himself. In memory of the kid, the detective quit his job (I think) and started backpacking again.
His parents died (for some unknown reason) and he was taken in by his grandparents. He killed his grandparents in a fit of passion. http://www.mangago.me/read-manga/yomigaeru_kiro/mh/c001/15/
And the detective wanted the kid to turn himself in and 'fix everything'.
http://www.mangago.me/read-manga/yomigaeru_kiro/mh/c001/16/
Can someone explain the plot