Tetsuo Tosu never expected his daughter, Reika, to be a victim of domestic violence. Suspecting that her yakuza boyfriend, Nobuto Matori, is the perpetrator, Tetsuo decides to investigate him and his associates. But as he searches for evidence at Reika's apartment, the father learns that his daughter is not the first victim and that a worse fate awaits her. When he is discovered by Nobuto, Tetsuo ends up killing him in the ensuing fight. Assisted by his wife, Tetsuo hides the body in fear of retaliation from the yakuza. To add to the misfortune of the Tosu family, Nobuto happens to be the son of the mafia's head. Considered to be prime suspects behind Nobuto's disappearance, Tetsuo and his wife will have to deploy all their ingenuity to protect their daughter from the yakuza and their ruthless investigation.

During his summer vacation, middle school student Nagasumi Michishio travels to the Seto Inland Sea. One day, while swimming at Mio Sun Beach, his leg suddenly cramps. No one is close enough to notice his desperate screams for help, and so he sinks into the ocean, where he is left to drown alone. Just as he loses consciousness, however, a mermaid appears and saves his life. That night, Nagasumi is visited by his savior, a girl who introduces herself as Sun Seto—a mermaid from a yakuza family. As it turns out, under mermaid law, a mermaid whose identity is revealed to a human must be punished by execution. To avoid this harrowing outcome, the Seto family propose a solution: Nagasumi must marry Sun or die at the hands of Gouzaburou, Sun's father and boss of the Seto clan. Faced with no other option, Nagasumi takes her hand in marriage. Now, the newlyweds face the difficult task of keeping their relationship secret. Between Gouzaburou's unending attempts on Nagasumi's life and the eccentric antics of a slew of antagonists, a genuine and innocent love blossoms between the pair as they adapt to their new life.

During the late 19th century, Great Britain has become the greatest empire the world has ever known. Hidden within its success, the nation's rigid economic hierarchy dictates the value of one's life solely on status and wealth. To no surprise, the system favors the aristocracy at the top and renders it impossible for the working class to ascend the ranks. William James Moriarty, the second son of the Moriarty household, lives as a regular noble while also being a consultant for the common folk to give them a hand and solve their problems. However, deep inside him lies a desire to destroy the current structure that dominates British society and those who benefit from it. Alongside his brothers Albert and Louis, William will do anything it takes to change the filthy world he lives in—even if blood must be spilled.

Great Britain is ablaze with news of a so-called "Lord of Crime," a criminal mastermind responsible for the downfall of several unruly nobles. In truth, the Lord of Crime is not an individual, but rather a group consisting of William James Moriarty and his two brothers, Louis and Albert. Together, they wish to destroy everything rotten about their current world and create a new, fair society for all. To accomplish their goal, they must commit criminal acts, which the great detective Sherlock Holmes and his partner, John H. Watson, cannot abide by. A dangerous cat and mouse game begins between the Lord of Crime and Sherlock, with each trying to outwit the other. Yet Sherlock, despite his skills, has no idea that his foe is right under his nose. Involved with both parties is a woman named Irene Adler, who is as beautiful as she is cunning. No stranger to scandal, Irene has embroiled herself in one that may be too big even for her, as the classified documents she stole could shake the very foundations of the British Empire. Can Irene be saved, or does a deadly future await her?