Traveling in search of the rare ingredient, “sky fish” Meliodas and Hawk arrive at a palace that floats above the clouds. The people there are busy preparing a ceremony, meant to protect their home from a ferocious beast that awakens once every 3,000 years. But before the ritual is complete, the Six Knights of Black—a Demon Clan army—removes the seal on the beast, threatening the lives of everyone in the Sky Palace.
In the distant technological future, civilization has reached its ultimate Net-based form. An "infection" in the past caused the automated systems to spiral out of order, resulting in a multi-leveled city structure that replicates itself infinitely in all directions. Now humanity has lost access to the city's controls, and is hunted down and purged by the defense system known as the Safeguard. In a tiny corner of the city, a little enclave known as the Electro-Fishers is facing eventual extinction, trapped between the threat of the Safeguard and dwindling food supplies. A girl named Zuru goes on a journey to find food for her village, only to inadvertently cause doom when an observation tower senses her and summons a Safeguard pack to eliminate the threat. With her companions dead and all escape routes blocked, the only thing that can save her now is the sudden arrival of Killy the Wanderer, on his quest for the Net Terminal Genes, the key to restoring order to the world.
Love is blooming at Sakuragaoka High School. Natsuki Enomoto has finally mustered the courage to confess to her childhood friend, Yuu Setoguchi. However, in the final moments of her confession, an embarrassed Natsuki passes it off as a "practice confession." Oblivious to her true feelings and struggling with his own, Yuu promises to support Natsuki in her quest for love. While Natsuki deals with her failed confession, fellow classmate Koyuki Ayase struggles with his own feelings for Natsuki. Despite his timidness, he is determined to win over her heart.