In the near future, there are people with special power called Sevens. It seems like the country has maintained peace by a company called Sumeragi Group, but they are the one who captured the people with the special power by force and did inhumane experiments. Gunvolt, the main character who has the power of Sevens, used to be in Feather, a resistant group who fought against the Sumeragi Group. The story begins with meeting the heroine, Sian.
Living by the sea, protecting the sea, and journeying across the sea... They're the Blue Mermaids!
Because of a shift in tectonic plates around 100 years ago, Japan lost much of its area to submergence. To preserve Japan's territories, coastal cities sprung up, one after the other. Eventually, they became marine cities, and along with the expansion of sea-lanes to connect them, the need arose for a sizable staff to protect the seas.
At the same time, maritime jobs were becoming more popular among women. Hence, the Blue Mermaids, tasked with keeping the seas safe, had become every female student's dream job. This is when childhood friends Akeno Misaki and Moeka China enroll in a marine high school in Yokosuka, along with other girls who share their goal of becoming a Blue Mermaid.
With the invention of anti-gravitational shoes known as Grav-Shoes, the ability to fly freely has become an everyday experience for the people inhabiting a four-island archipelago south of Japan. This invention has brought the people new ways of living and also a new sport known as "Flying Circus," where participants gain points by either touching floating buoys or their opponent's back.
The gullible and clumsy Asuka Kurashina, newly transferred to Kunahama High School, enters this world of flight unknowingly when she is able to pull off a difficult maneuver the first time she participates in a Flying Circus match. Eventually, this leads her to join her school’s Flying Circus club. Led by their coach, Masaya Hinata, their members consist of the experienced Misaki Tobisawa and her overprotective friend, Mashiro Arisaka. Ao no Kanata no Four Rhythm follows this rookie group soaring high above the skies and toward their dreams, armed only with their unwavering passion against an uncertain future.
In the distant future, a majority of humans have left the Earth, and the Galactic Alliance of Humanity is founded to guide exploration and ensure the prosperity of mankind. However, a significant threat arises in the form of strange creatures called Hideauze, resulting in an interstellar war to prevent humanity's extinction. Armed with Chamber, an autonomous robot, 16-year-old lieutenant Ledo of the Galactic Alliance joins the battle against the monsters. In an unfortunate turn of events, Ledo loses control during the battle and is cast out to the far reaches of space, crash-landing on a waterlogged Earth.
On the blue planet, Gargantia—a large fleet of scavenger ships—comes across Chamber and retrieves it from the ocean, thinking they have salvaged something of value. Mistaking their actions for hostility, Ledo sneaks aboard and takes a young messenger girl named Amy hostage, only to realize that the residents of Gargantia are not as dangerous as he had believed. Faced with uncertainty, and unable to communicate with his comrades in space, Ledo attempts to get his bearings and acclimate to a new lifestyle. But his peaceful days are about to be short-lived, as there is more to this ocean-covered planet than meets the eye.
On the front lines of the war, there is a little girl. Blond hair, blue eyes, and porcelain white skin, she commands her squad with a lisping voice. Her name is Tanya Degurechaff. While in reality, she is one of Japan's most elite salary-men, reborn as a little girl after angering a mysterious being who calls himself God. This little girl, who prioritizes efficiency and her own career over anything else, will become the most dangerous being amongst the sorcerers of the imperial army.