From MangaHelpers: 1x1t5l

Tanaka Ichirou's life is shaping up to be pretty horrible. Despite his hard work, he's not very good at his company job. In exchange for keeping his job, he is coerced into marrying an unwilling girl. The girl was impregnated by her lover, a man who was already engaged, and the company wanted to avoid a scandal. Now everyone abuses Tanaka at work, and he goes home to take out his frustration on his unhappy wife. To top it all off, some people are planning to put an end to the whole situation by having him killed.

It is this last problem that brings him to the attention of a highly skilled ninja hitman, Shadow. Shadow is fleeing his criminal organization in search of a peaceful, normal, happy life. When he witnesses Tanaka's final moments, he decides to take the unfortunate man's life as his own. Will Shadow be able to find some kind of happiness in Tanaka's life, and will he be able to avoid his own past? [tethysdust]