Father Mateo is a Jesuit priest working as a missionary in 16th century Japan. His personal mission is to preach to the merchant and lower classes, which distresses the upper classes who are distressed by his opinion that all are equal in the sight of God. He is a fairly saintly man. He is assisted by Matsui Hiro, whose real name is Hattori Hiro, and officially acts as translator and secretary. Father Mateo speaks Japanese, but explains that he likes to have Hiro along since he sometimes misses nuances. Hiro is actually a shinobi (aka ninja) employed by a mysterious client to protect Father Mateo. How much Father Mateo knows about this arrangement is unclear. The two are sometimes called upon to solve mysteries.