Monsters is a oneshot manga by Eiichiro Oda, originally published in the 1994 Shonen Jump Autumn Special. It is canonically part of One Piece, although this was not revealed until One Piece's Thriller Bark Arc and confirmed in SBS Volume 47.
The story starts with the narrator telling the story of a dragon's horn that was stolen 10 years ago. It was the only such thing existing and was said to have wondrous powers. But no one still remembers the incident.
In the next scene, someone is being kicked out of a restaurant due to him being unable to pay for his meal. Meanwhile in another restaurant, the waitress Flare recognizes the famous swordsman Cyrano, whom she had encountered before. Their conversation is interrupted by a mad looking guy staring through the window on Cyrano's meal with his stomach rumbling loudly. After Flare gave him something to eat, the samurai Ryuma states that this has been his first meal in five days and offers his gratitude to the young waitress. He claims that a samurai owes his life to his life-saver, but is ignored by Flare who is talking to Cyrano again. When the latter leaves the restaurant, his sword accidentally touches Ryuma's, which the samurai takes as an heavy insult. He challenges Cyrano to a duel-to-the-death. As the two swordsmen engage each other, Ryuma slashes a copper Buddha easily in half. Right before the fight is escalating, Flare tells Ryuma to stop. Much to the viewer's surprise, he follows her order immediately. At the same time, another swordsman called D.R. arrives at the city.
Later on, Ryuma is seen sitting at the bar learning from the bartender the story of Flare's hometown. Seven years ago, the village in the mountains Flare lived in was suddenly attacked by a huge dragon. The only survivors were Flare and Cyrano, who saved the girl from the flames of the burning village. This event established Cyrano's reputation as hero and great swordsman. After listening to her story, Ryuma has great respect for Flare, who he thinks of as a very brave girl. He then has a chat with the bartender about "The King", who is supposed to be the greatest swordsman alive, with Ryuma expressing the intention of fighting "The King" one day, since he believes "The King" has the soul of a real fighter. After finishing his meal, Ryuma decides to continue traveling.
As he is walking down the street, Ryuma's sheath is seen yet again knocking against D.R.'s. As Ryuma is calling the man for touching his sword, D.R. suddenly starts to scream and accuses Ryuma of stabbing him. While Ryuma is completely confused by the situation, D.R. further accuses him of wanting the Dragon Horn he allegedly stole. He then decides to blow the horn in order to summon a dragon to destroy the whole town. He then breaks the horn and apparently dies. The surrounding crowd blames Ryuma for their misfortune and becomes violent towards him when Flare appears. Much to Ryuma's surprise, she gives him a slap in the face and calls him a murderer for causing the situation. Remembering her former encounter to a dragon, she states that he can't imagine how terrible a dragon really is, because if he did, he wouldn't have done something that stupid. Ryuma still doesn't get the situation at all and refuses to take responsibility. Cyrano however confirms that the horn is real and asks everyone to leave the town immediately. He wants to fight with the dragon on his own though Flare is begging him not to do so. Although Ryuma is stating that it is stupid to risk his life just for a few houses, Cyrano sticks to his decision. The town's inhabitants are still mad at Ryuma, telling him to commit Harakiri, but he is stopped by Cyrano.
Later on, all the town's inhabitants have fled except for Flare and Ryuma who are talking in the restaurant. Ryuma doesn't understand why Flare is so frightened since Cyrano is supposed to be a really good swordsman. When Flare, haunted by her memories, storms off in order to stop Cyrano, Ryuma follows her outside. As they stop running and begin arguing again, Ryuma suddenly freezes in shock and tries to prevent Flare from turning around. As he fails, the two of them watch Cyrano helping D.R. looting the town.
It turns out that Cyrano and D.R. were partners from the start and they trapped Ryuma in order to blame him for the incident with the dragon. It is revealed that they were also responsible for the attack that happened seven years ago. Back then, D.R. summoned a dragon in order to destroy the village with the Dragon Horn that Cyrano had stolen from his own subordinates. Flare encountered Cyrano in the middle of the flames and begged him for help. Instead, he killed her father while she was unconscious and decided to use Flare in order to gain prestige for his name.
As Ryuma and Flare are hiding behind a house, Flare tells him that staying any longer would be suicide as a dragon was still coming to destroy the town. She tells him to immediately leave the town, but instead of fleeing herself she returns to her place and breaks down in tears. Ryuma, who has followed her again and witnesses her tears and screams from outside, runs off to confront Cyrano and D.R. When he faces the two criminals, he states that he knew someone who would knock against another sword so thoughtlessly could never have a true fighter's soul. As Cyrano mocks Ryuma and finally attacks him, the latter kills him with one strike. D.R., frightened by this turn of events, pleads for his life by offering Ryuma the real Dragon Horn in order to stop the dragon from destroying the town. Ryuma just tells him that he is not an inhabitant of this town and draws his sword. Just as the dragon arrives, Flare is seen sitting on a rooftop, preparing to die together with the town. By then, she realizes Ryuma is confronting the dragon. The samurai jumps from a building and beheads the beast with one mighty swing.
As Ryuma is leaving the town after the battle, he meets Flare again. When he passes her by, he tells her to stop thinking about dying as he believes dying isn't fun at all. When Flare asks him why he isn't famous at all, he just states that reputation isn't important in a fight but what you are protecting with it. As a farewell, he tells Flare that she owes one to him now and that he will return in order to have a meal. Later on, as the inhabitants of the town are returning to their houses, Flare learns from the bartender that Ryuma is actually called "The King" and that this name was given to him by those he had saved. He is believed to be the world's best swordsman.