"Monster Musume no Iru Nichijō SP" is a special episode that expands on the comedic and romantic elements of the original "Monster Musume no Iru Nichijō" series. The story centers around Kimihito Kurusu, a young man who becomes an involuntary host to several supernatural girls through the Interspecies Cultural Exchange Act, a government program designed to integrate mythical creatures into human society. Each of these girls represents a different species, including a lamia, a harpy, a centaur, a slime, a mermaid, and a spider-like arachne.

The special episode explores the daily challenges and humorous situations that arise from Kimihito's cohabitation with these unique individuals. Miia, the lamia, is particularly affectionate and often competes with the others for Kimihito's attention. Papi, the harpy, is carefree and childlike, while Centorea, the centaur, is formal and chivalrous. Suu, the slime, is curious and adaptable, Rachnera, the arachne, is confident and teasing, and Mero, the mermaid, is romantic and melodramatic. The interactions between these characters and Kimihito create a blend of comedy, lighthearted drama, and occasional romantic tension.

The plot of the special episode revolves around a series of vignettes that highlight the quirks and personalities of each character. These segments often involve misunderstandings, cultural clashes, and the girls' attempts to adapt to human life while navigating their feelings for Kimihito. The narrative maintains a light-hearted tone, focusing on the absurdity of the situations and the comedic potential of interspecies relationships. The special episode does not significantly advance the overarching plot of the series but instead serves as an entertaining extension of the original premise, offering fans additional moments of humor and character interaction.