Monster Samurai follows the trials and tribulations of Midomaru, a cold-hearted assassin who is turned into an ogre one day by a mysterious figure. As Monster Samurai, Midomaru must now save one thousand people to atone for his villainous past, but will be eternally cursed should anybody else fall to his blade. But even as he laments this punishment, an opportunity arises – a young boy, pursued by a pack of wicked Gabureez ninja! Angry at having his rest disturbed, Monster Samurai makes short work of the ninja, but an even greater trial lies just ahead… the boy, Kai, is greatly impressed and attaches himself to the horrified Monster Samurai as his apprentice!

As the story progresses, the unlikely pair form an even more unlikely bond. On a quest to rescue his mother from the evil Lord Bah’Shaan, who seeks to rule the world from his moving citadel, Kai is desperate to learn the ways of a warrior from his new idol, Monster Samurai. Monster Samurai himself, however, just has his hands full playing father figure to the excitable youngster! Together, the two of them will face unbelievable adventures and overwhelming odds in their quest to save the world, one person at a time!