After conquering the galaxy, mankind has reached its apex and is now on a steady decline. The resources of the universe are diminishing, and around 500 billion humans begin to return home to Earth. Thus begins a war between the various factions of humans for control of the planet. Eventually, an authoritarian government known as the Gaia Sanction prevents the re-population of Earth. Captain Herlock was one of the Elite Wing who was tasked by the Gaia Sanction to defend the Earth. His fleet of ships was unstoppable under his command. However, the Gaia Sanction allows a diplomatic elite to immigrate to Earth, which angers Herlock. He goes rogue and fires upon the diplomats and the rest of his fleet. He decides to unleash dark matter on the planet to make it uninhabitable, but also becomes engulfed in it which immortalizes him. One hundred years have passed, and the legendary Captain Herlock is still at large, with only the Arcadia under his control. The Gaia Sanction continues with their plans for control over the Earth, while hiding its true state. But Captain Herlock is preparing for one final showdown with them, which will determine the fate of the world!

After the final battle against Gaito, Sara had given her life to remain with him. The sea world is now at peace, but only with six mermaid princesses. While Caren, Noel, and Coco have returned to their kingdoms, it should be time for Lucia, Hanon, and Rina to do the same. After all, Mitsuki Tarou is leaving for Germany, and Doumoto Kaito is heading to Hawaii. Lucia is devestated, and when things couldn't get any worse, Kaito is lost at sea! Worst of all, a strange angel named Michel has appeared, wanting the mermaids to join him. But on the lighter side, Sara's spirit tells Lucia of Sara's successor, a young mermaid named Seira. The mermaids are in for another adventure!

Yamato Akitsuki travels to Tokyo alone to study in one of the high schools located within the area. He lives in with his aunt who operates a public bath solely for the ladies in the local district and begins his normal high-school life. One day, he chances upon a girl in school and is immediately mesmerized by her beauty. He is shocked when he realizes later that the girl, Suzuka, is actually living next door to him. From then on, Yamato's ordinary life begins to change little by little.

After swearing off music due to an incident at the middle school regional concert band competition, euphonist Kumiko Oumae enters high school hoping for a fresh start. As fate would have it, she ends up being surrounded by people with an interest in the high school brass band. Kumiko finds the motivation she needs to make music once more with the help of her bandmates, some of whom are new like novice tubist Hazuki Katou; veteran contrabassist Sapphire Kawashima; and band vice president and fellow euphonist Asuka Tanaka. Others are old friends, like Kumiko's childhood friend and hornist-turned-trombonist Shuuichi Tsukamoto, and trumpeter and bandmate from middle school, Reina Kousaka. However, in the band itself, chaos reigns supreme. Despite their intention to qualify for the national band competition, as they currently are, just competing in the local festival will be a challenge—unless the new band advisor Noboru Taki does something about it. [Written by MAL Rewrite]