"Happy World!" centers around Takurou Kamiya, a high school student who experiences a series of unfortunate events, leading him to believe he is cursed. After a particularly bad day, he encounters a mysterious girl named Liru, who claims to be a "happiness bringer" sent to improve his life. Liru explains that she is part of an organization dedicated to spreading happiness, and her mission is to help Takurou overcome his misfortunes. However, her methods are often unconventional and chaotic, creating more comedic mishaps than actual solutions.

As the story progresses, Takurou's life becomes increasingly intertwined with Liru and her eccentric world. He meets other happiness bringers, each with their own quirks and approaches to spreading joy. These include the stoic and serious Shia, the mischievous and playful Miu, and the enigmatic leader of the organization, Rin. Despite their efforts, Takurou's life remains a mix of absurdity and misfortune, with Liru's presence often exacerbating the chaos.

The narrative explores themes of fate, happiness, and the unpredictability of life. Takurou's struggles with his perceived curse and his growing relationships with the happiness bringers form the core of the story. The series balances humor with moments of introspection, as Takurou gradually learns to navigate his challenges and find meaning in his experiences. The interactions between the characters drive much of the plot, with Liru's relentless optimism and Takurou's skepticism creating a dynamic that fuels both comedy and character development.