Youkou no Ken

Feudal Japan was a land beset with warfare and strife. Families were torn asunder and siblings were divided by conflict and chaos. Matagoro Yoshimura has been hired by the feudal lords to seek out a treasure of incredible wealth —but there's a catch... the only way to find this cache is by gathering the pieces of the map that have been hidden on the backs of female ninja scattered throughout the land. The only time these hidden maps will reveal themselves is when the ninja carrying it is in the throes of passion. Is Yoshimura "up" to the task of putting together the puzzle pieces and finding the treasure?

(Source: AniDB)