A few months after the events of Steins;Gate, Rintarou Okabe and his group of friends are invited to tag along with their acquaintance Faris NyanNyan, who is participating in a Rai-Net battle event in the United States. There, they meet up with an old colleague: Kurisu Makise, who has been recalling fragmented memories of events that happened in the other world lines in the form of dreams. She confronts Okabe, questioning him as to whether these events—particularly the incidents between the two of them—did indeed happen. [Written by MAL Rewrite]

An injured man is found in the woods by a girl named Eruruu, and everything about him is mysterious. Without knowledge of his past nor even his own name, he is welcomed to Eruruu's home and is given the name Hakuoro by her grandmother, and younger sister, Aruruu. While the inhabitants of the village have large ears and tails, Hakuoro's defining physical trait is quite different as he has neither ears nor tail, but only a mask that he cannot remove. Soon after he becomes a part of the villagers' lives, a revolution against the tyrannical emperor of the land begins, and the conflict finds its way to his new home. Hakuoro must do whatever he can to save the people and the village that he has come to love, all while uncovering the mysteries that shroud his past. [Written by MAL Rewrite]

The wandering knight, Bedivere, reaches the end of his journey. The year is A.D. 1273 in Jerusalem. The Holy Land has been transformed into a massive desert, the people have been forced out of their homes, and three major powers wage war with each other in this wasteland. The Knights of the Round Table come together to protect the Holy City and their Lion King. With the whole of his kingdom summoned into a strange land, Ozymandias, the Sun-King, quietly plots against the tyranny of this bizarre realm. The mountain people, protectors of those who were stripped of their land, await their chance at rebellion. In order to fulfill his mission, Bedivere heads for the Holy City where the Lion King rules. There he meets humanity’s final Master, Ritsuka Fujimaru, who has come to Jerusalem, accompanied by his Demi-Servant, Mash Kyrielight, in their quest to restore human history. Bedivere joins with Fujimaru and his companions to undertake this quest.

High school student Haru Yoshioka is bored with the monotony of life. One day, she saves Prince Lune of the Cat Kingdom from being run over by a truck. As a token of gratitude, the Cat King sends her "presents" and invites her to the Cat Kingdom to become Lune's wife. Haru's inability to properly communicate with the cats leads to the misunderstanding that she has accepted the proposal.

As Haru ponders on ways to escape the predicament, a mysterious voice instructs her to search for the Cat Bureau. However, not long after she finally arrives at the bureau, a horde of cats swarms in and forcibly takes her to the Cat Kingdom, along with a member of the Cat Bureau. Concerned for their safety, owner of the Cat Bureau, Baron Humbert von Gikkingen, follows close behind.

The more Haru immerses herself in the activities of the Cat Kingdom, the more cat-like she becomes. To her dismay, she soon learns that, unless she can find her true self, she may become a cat permanently. Haru's adventures in the world of cats lead her down a path to self-discovery, allowing her to return as a more confident person.