Set in Japan during the '20s, Mitsuyaku is the story of two childhood friends from very different walks of life who grow up to become lovers. Yuuichi is an ambitious type from humble beginnings, working his way towards success. His beloved Iori is the scion of an important family – a beautiful, dark-haired creature with a gentle nature.

Unfortunately, one of the aspects of belonging to an important lineage is marriage. Will the hero have the guts to stay with the love of his life, or will he succumb to his family's pressure to shore up the dwindling finances by marrying rich?