A.D. 2087 - the United Lunar Empire Giganos wages war on the Earth Federation Military to take control of the planet and establish a "rebirth" of the human race. During an invasion of a colony by Giganos' forces, three civilian men stumble upon a trio of top-secret Metal Armor units called "Dragonars" and pilot them to combat the enemy forces.

Shiina Aki is constantly being treated like a girl due to his feminine looks so he decides to move to Tokyo to attend middle school in an attempt to change himself. However, what awaits him in his new home, Sunohara-sou, is the kind-hearted caretaker, Sunohara Ayaka. Along with the three female members of Aki's new middle school's student council, Yukimoto Yuzu, Yamanashi Sumire, and Kazami Yuri. And so begins Aki's new life in Tokyo living with four girls.

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]

Catholic schoolgirl Inaho discovers that a vampire, Mosquiton, is feeding off of her classmates. So she stakes him, but he is revived after her blood comes in contact with the his remains. Mosquiton becomes her slave and also a history teacher. Together, along with Yuuki and Honou, the unlikely duo have many escapades and adventures. One of Inaho's main goals is to find the mythical O-Part to make some money! (Source: AnimeNfo)