Nail and Thorn

Nanao Shuuji left his hometown where his family runs a well-known chain of supermarkets, and became a part-time teacher at a cram school. One day the son of his father's mistress, Takatou Kazuomi, appears before him. Shuuji once confronted Kazuomi's mother, and also injured Kazuomi as well. Kazuomi made use of Shuuji's weakness in order to keep getting financial aid from his father, and ordered Shuuji to help him, while blackmailing Shuuji with a particular photo...?!