Don't Call it Mystery (Omnibus), Vol. 3-4

When Totonou goes to Hiroshima, his only goal is to finally see an art exhibit (and maybe run into his fugitive best friend). But instead, Totonou gets dragged into a battle of wits. Kariatsumari Shioji is a teenage girl who needs his help. Shioji and her cousins have been named as beneficiaries in their grandfather's will, but they'll only receive their share if they can solve a cryptic puzzle. If that wasn't enough, Shioji suspects that her cousins are willing to kill in order to win. After all, last time there was an inheritance battle in the family, a whole generation ended up getting wiped out. Can Totonou unravel the mystery before there's another bloodbath?