"Mini Ika-Musume" side-story episodes.

After surviving the brutal challenges of Idofront, Riko now possesses a White Whistle, allowing her to descend into the Abyss's sixth layer—The Capital of the Unreturned. Alongside Reg and Nanachi, Riko begins to explore the uncharted domain, where the ruins of the promised Golden City are located. As the trio starts to adapt to the harsh environment, they soon encounter dangerous creatures and treacherous landscapes. Their expedition leads them to a village inhabited by strange beings known as "hollows." Despite the creeping sense of unease that welcomes them, the three venture onward to uncover the mysteries of the settlement and long-lost legacies of the forgotten adventurers who once descended into the horrors of the unexplored Abyss.

On Halloween night, a young chimney sweep named Lubicchi befriends a man magically conjured out of garbage. Giving him the name Poupelle, Lubicchi is able to recreate a bond he lost when his father died to the tyranny of the Inquisitors—an oppressive force that aims to suppress outside knowledge and the town's true history. But their newfound relationship soon captures a lot of attention, and the threat of the Inquisitors and even the criticism of a few townspeople will test the duo's friendship. In spite of this, Poupelle has struck a particular chord with Lubicchi, and the two conspire to bring the truth to the surface and clear the chimney smoke that clouds the town's skies.

Shoutarou Hidari, a hard-boiled private investigator from the Narumi Detective Agency, lives for the sake of protecting the peace in the city of Fuuto alongside his inquisitive partner, Philip. One fine night, Shoutarou encounters a mysterious woman named Tokime, who is soon linked to several cases of homicide and theft. Shoutarou's investigation leads him to people using "Gaia Memories"—memory units similar to thumb drives holding vast amounts of data—to turn into formidable beasts called "Dopants." While the majority of Gaia users were put down by the detective team in the past, a number of them still remain, posing a great threat to public safety. Once more, Shoutarou and Philip, Gaia users on the side of justice, must fight as Kamen Rider W—the renowned hero safeguarding Fuuto—to eradicate the nest of villainous Dopants, all while uncovering Tokime's true identity and confronting the long-forgotten past.