一起发生于丰后关前藩的意外，让磐音失去了两个发小。伤心的磐音决意离开家乡，于是告别了未婚妻奈绪，成为了一名漂泊无主的武士。磐音在江户的一间长屋中开始了自己的新生活，经屋主金兵卫的介绍，磐音白天在鳗鱼店做伙计，晚上在当铺老板今津的屋里做保镖。他温柔的秉性和出色的剑道慢慢赢得了周围人的信任，也虏获了金卫兵女儿的芳心。就在这时，磐音却被卷入了幕府流通的新货币阴谋之中。为了保护在江户遇到的这些重要之人，磐音必须将哀痛搁置心中，与恶势力进行对抗。

Iwane Sakazaki returns to his homeland and gets caught in the middle of an incident that results in the tragic death of two of his best friends from childhood. He decides to leave his domain, parting with his fiancée Nao, and becomes a vagrant masterless samurai with nothing more to lose. Iwane drifts to Edo, filleting eels during the day and working as a bodyguard at night for Imazuya, a reputable money exchanger. He gradually wins the trust of the people around him because of his mellow nature, the chivalrous way he treats everyone with courtesy, and his skills of swords. One day, he learns that Imazuya is being targeted in a conspiracy to sabotage a new monetary system implemented by the government, and Iwane decides to protect the people who have given him support.

江戸時代、ある事件により2人の幼馴染と婚約者を失った坂崎磐音 (いわね)。九州から江戸へ移った彼は、昼はうなぎ屋で働き、夜は両替商の用心棒という生活を送り始める。穏やかな人柄により周囲の人々から信頼を持たれる磐音だが、あるとき新貨幣をめぐる陰謀に巻き込まれてしまう。