
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. [Written by MAL Rewrite]

Modern Germany is plagued by an outbreak of "Amalgams." Existing solely to wreak havoc, these cybernetic entities spawn from rotting flesh and can fuse with technology to gain new abilities. With society left in the wake of their destruction, the Xenogenesis Assault Team (XAT) is formed to suppress the threat. Alongside its primary mission to protect against the Amalgam attacks, the organization is also researching the newly discovered "amalgamated" humans which possess rational thought and are far deadlier than their non-sentient counterparts. Joseph Jobson is one such amalgamated human who has full control over his powers. Although successful in his line of work as a lone warrior, an unfortunate encounter with the recently-turned-Amalgam Gerd Frentzen makes him a priority target of the XAT. As he eludes the organization and seeks new allies, Joseph is transformed into the Blassreiter—a being heralded as the strongest Amalgam in existence. Now, he must fight back with his newfound powers to uncover the truth behind not only his past, but also the entire Amalgam conflict. [Written by MAL Rewrite]

Although traumatized by witnessing the murder of his parents by one of the Choushuu, Ichimura Tetsunosuke's thirst for revenge lead him to desire strength. At the age of 15, Tetsunosuke approached the Shinsengumi, wanting to become one of its members. However, Tetsunosuke lacked the skill, mind and will to emotionlessly cut down whoever threatened peace and the Shinsengumi. Even with the support of his brother Tatsunosuke and his newfound friends of the Shinsengumi, little did Tetsunosuke know the blood and pain he would have to face being part of this historical group.

In 1614, two rival ninja clans back opposing sons of Hidetada Tokugawa for the shogunate. They send their finest warriors to battle to the death, with the victor's clan ruling for a millennium.