Nijiiro Days chronicles the everyday routines of four high school students: Natsuki Hashiba, a hopeless romantic; Keiichi Katakura, a sadist; Tomoya Matsunaga, a playboy; and Tsuyoshi Naoe, an intelligent otaku. Despite their constant academic struggles, they spend their time fooling around and experiencing romantic mishaps. On Christmas Eve, however, Natsuki is broken up with by his girlfriend. While weeping in solitude, he receives a pack of tissues from a girl dressed in a Santa Claus costume. In exchange, he gives her the scarf that was originally intended for his ex-girlfriend. When the holidays conclude and Natsuki returns to school, he discovers that the girl is named Anna Kobayakawa—who happens to attend the same school as him! Having fallen in love with Anna, Natsuki strives to get closer to her, even though his friends interfere with his plans.