The plot of Nijiiro Days revolves around the eventful lives and amorous affairs of four male high school students. Natsuki Hashiba's idealistic notion of love leads him astray, while Tomoya Matsunaga's self-centered attitude lands him in a relationship with multiple girls. Keiichi Katakura's unconventional sexual preferences are hard to ignore, and Tsuyoshi Naoe's partner has a penchant for cosplay. On Christmas Eve, Natsuki's girlfriend ends their relationship, causing him to break down in tears on the street. He is comforted by a girl dressed as Santa Claus, who turns out to be Anna Kobayakawa, a fellow student at his high school. However, Natsuki's pursuit of Anna is hindered by the interference of his meddlesome friends, who are grappling with their own romantic interests.