SARAZANMAI

The stage is Asakusa. One day, three 2nd-year middle school students — Kazuki Yasaka, Tooi Kuji, and Enta Jinnai — meet a mysterious kappa-like creature named Keppi, who forcibly takes their shirikodama (a mythical organ kappa steal through a person's anus) and transforms the boys into kappa. Keppi informs them, "If you wish to return to your former selves, you must connect in 'that way' and bring me the shirikodama of zombies." Will the three boys be able to connect as they steal shirikodama from zombies?! Meanwhile, two policemen — Reo Niiboshi and Mabu Akutsu — are up to something at the police box where they are stationed. (Source: Official Website) — Description from AniList community (anilist.co).