弟弟李鲸被暗玉浊气侵蚀危在旦夕，父亲李鲲离奇失踪，走投无路的少年李鲥，在家传鲲佩的指引下，召唤出来自鲲域的锦鲤仙李鲤。为报李家先祖的救命之恩，傲娇灵动的锦鲤仙与执着坚定的人间少年，结伴踏上了一场险象环生的大冒险。
故事以史上首个自律人型AI「Vivy」为主角，是以「歌声带给众人幸福」为使命的「歌姬」。某日自称来自100年后的AI「松本（マツモト）」突然出现，并表示自己将与Vivy共同修正历史、避免100年后人类与AI发生战争，使Vivy踏上破坏AI的旅途。
In the year 1524, the former Assassin Ezio Auditore retired to a villa in the countryside of Tuscany, with his wife Sofia and children Marcello and Flavia. While Ezio wrote in his study, Sofia asked him to watch Flavia while she and Marcello visit his sister Claudia. Shortly after, he left his study to tend their vineyards while Flavia played nearby the villa. In the midst of his harvest, he saw a stranger cloaked in black, talking to Flavia. Fearing the worst, Ezio hurriedly intercepted the stranger, though Flavia told him that she was no danger, as Ezio saw her necklace with the Assassin insignia...
《地球守护者21》作为前作的续篇，由角川书店、IMAGICA以普通版和限量版OVA的方式发行。全6卷。本作的舞台移到了2001年，是与前作不同的，更写实的故事。世界观也更加黑暗。
