On her birthday, Halloween, Kuromi finds herself feeling neglected as her friends are preoccupied with holiday preparations instead of celebrating with her. Frustrated, she wishes to leave her world behind, a plea that unexpectedly tears open a portal and sends her tumbling into the multiverse.

Lost in a chaotic network of parallel dimensions, Kuromi meets Nico, a mysterious young man who is also searching for a way home. Together, they form an uneasy alliance to navigate the bizarre realities they encounter. Their journey brings them face to face with a wide array of alternate versions of Kuromi herself. These variants include a fun-loving zombie Kuromi, a tragic villainess lady Kuromi, and a harried manga artist Kuromi struggling to meet a deadline. The pair also explore worlds such as a gothic realm ruled by a tyrannical Duke Baku and a toy universe populated by sentient plushies.

As they hop between dimensions, the original Kuromi learns that the multiverse is under threat from Dark Kuromi, a jealous, all-white doppelgänger who lives in a barren wasteland called the Darkness World. Dark Kuromi envies the original for having friends and plots to trap Nico's parents and take over Kuromi's life. Kuromi must rally her friends and unite the various versions of herself to confront this shadowy foe, uncover Nico's true identity, and fix the chaos she inadvertently unleashed so she can return to her own world and make amends with her friends before her birthday is over.