Marimo runs into Udonko, who is on her way to look for an unidentified flying object that crashed into a nearby park. While searching for the UFO, they meet up with Tomarin and Tanaka-san, who points them in the direction of some peculiar "playground equipment."
Simply put, it's one of those Nessie-esque 
mysterious disk things like a UFO.
Es una de esas cosas extrañas como 
los OVNI o el monstruo del lago Ness.
Basicamente, é um daqueles monstros
tipo Nessie, coisa como um OVNI.