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.

When a handsome vampire finds a kitten on the verge of death, he cannot help but take pity on the poor creature and save its life the only way he knows how: by offering it some of his blood and granting it eternal life as a vampire cat. The kitten—now known as Nyanpire—does his best to acquire blood and other red substances for his consumption. At the same time, he must also manage the difficulties of being a household pet and keeping up with the other strange cats around the neighborhood.

In the far future, the elevated irradiation from the sun has destroyed the environment of the earth and the birthrate of humans has drastically decreased. The government controls society with an army of cloned soldiers called "Ideal Children (IC)". Sam Coyne is a trader in a desert. One day, he saves a beautiful woman Maya, who has been chased by Theseus, a corps of IC. He shelters her in his trade ship, but the destroyers of Theseus surround Sam and Maya.

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.