Second part of Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka IV.

Having died in a car accident, Tsubasa Kurata—an otaku from modern Japan—is reborn in the Fremmevilla Kingdom, a medieval world where powerful mechs called Silhouette Knights are used to fight horrific demonic beasts. Born into a noble family under the name of Ernesti Echevarria and bestowed with prodigious magical abilities, he enrolls into Royal Laihaila Academy. This school of magic trains young men and women on how to pilot the Silhouette Knights, prepping them to protect the kingdom from threats, both demonic and human. Ernesti teams up with the twins named Adeltrud and Archid Olter with the goal to create his own Silhouette Knight one day, a feat unheard of for several centuries. [Written by MAL Rewrite]

Due to the incredible success of the Sibyl System, Japan has begun exporting the technology to other countries with the hope that it will one day be used all around the world. In order to test its effectiveness in a foreign location, the war-torn state of the South East Asian Union (SEAUn) decides to implement the system, hoping to bring peace and stability to the town of Shambala Float and keep the population in check.

However, a group of anti-Sibyl terrorists arrive in Japan, and the Ministry of Welfare's Public Safety Bureau discovers significant evidence that the invaders are being aided by Shinya Kougami, a former Enforcer who went rogue. Because of their past relationship, Akane Tsunemori is sent to SEAUn to bring him back, but with their last meeting years in the past, their reunion might not go quite as planned.

The year is 2027, and the city of Kyoto has undergone tremendous technological advancement. Within the city lives Naomi Katagaki, a socially awkward and introverted boy with a love for books, and Ruri Ichigyou, a girl with a cold personality who is often blunt with people, but shares his love for reading. Despite having similar interests, Naomi is afraid to approach Ruri due to her unfriendly nature. One day, as Naomi goes out for a walk, a crimson aurora pierces through the sky for a brief moment before vanishing. Shortly after, he sees a three-legged crow and a mysterious hooded man who reveals himself to be Naomi from 10 years in the future, explaining that he has come to change an imminent tragic event that happens to Ruri shortly after they start dating. Initially taking his words with a grain of salt, present-day Naomi follows his future self's instructions and starts getting closer to Ruri, determined to save her. Hello World focuses on the present Naomi alongside himself from 10 years into the future. With the help of his future self, Naomi begins his preparations to save Ruri. Will he be able to change the future? [Written by MAL Rewrite]