Mio, a high school student known for her laziness and reluctance to invest fully in anything, spends her days after school in Ikebukuro with her childhood friend Arata and best friend Madoka. However, their dynamic shifts when Madoka confesses her love for Arata, prompting Mio to realize her own feelings for him. Fearing the repercussions on their friendship, Mio pretends to support them while grappling with her inner turmoil. Struggling to navigate this emotional maze, Mio distances herself from Arata, leading to a heated argument between them. As she confronts her avoidance of her emotions, she decides to reconcile with Arata and rushes to meet him amidst a rainstorm. Tragedy strikes as she gets involved in a car accident and awakens in a surreal world where trains glide on water and majestic whales swim alongside. Despite the familiarity of Ikebukuro in this strange realm, Mio encounters a enigmatic figure named GiMON, claiming to be the guide of this world. GiMON offers her a door that supposedly leads to any desired world, but a cryptic girl named Kiku intervenes, guiding Mio to the true exit at the Forgotten Things Counter. Here, one must relinquish a cherished lost memory and a burning desire to return home. Joined by GiMON and Kiku, Mio reaches the Counter, only to falter when she struggles to identify her treasured memory. Their vanishing leaves Mio stranded in a nostalgic yet unfamiliar Ikebukuro, prompting her to embark on a quest through the city's flood of memories in search of her lost treasure.