The world is suddenly introduced to the existence of liminal species—mythical beings such as lamias, harpies, centaurs, and merfolk—following a government initiative called the Cultural Exchange Act, which aims to integrate these non-human creatures into mainstream human society. The narrative follows Kimihito Kurusu, an ordinary, unassuming Japanese young man who inadvertently gets roped into the hosting program. Through a bureaucratic mix-up at the government's Interspecies Exchange Coordination office, overseen by the eccentric and overworked Smith, Kimihito is assigned to host Mero, a melancholic mermaid, but quickly finds his household overflowing as more monster girls are mistakenly sent to live under his roof.The primary conflict revolves around Kimihito attempting to survive the chaotic daily life of cohabitating with these high-maintenance, affection-starved, and often aggressive
supernatural roommates. Alongside Mero, the household includes Papi, a naive and airheaded harpy; Suu, a sentient and shape-shifting slime girl; Rachnera, a sadomasochistic arachne with a protective streak; and Centorea, a chivalrous centaur torn between her warrior code and her burgeoning, submissive attraction to Kimihito. The emotional core of the series focuses heavily on their host, Miia, an overly passionate lamia whose aggressive romantic pursuits frequently put Kimihito in awkward and life-threatening situations.Despite their different origins and instincts, the monster girls develop a fierce loyalty and genuine romantic affection for Kimihito, often clashing with one another in comedic turf wars for his attention. However, living arrangements are constantly threatened by the stringent laws of the exchange program. The overarching plot is driven by the looming threat of the "Interspecies Cultural Exchange Act" regulations, which strictly forbid intimate relations between humans and liminal beings. Whenever government inspectors, like the strict Mr. Dark, arrive to audit the household, Kimihito and his chaotic harem must go to great lengths to hide their unorthodox lifestyle and prove that humans and monsters can live in harmony.As the series progresses, the domestic disputes escalate into broader adventures involving supernatural threats and local misunderstandings, requiring the monster girls to utilize their unique abilities to protect Kimihito. Through every escalating crisis, medical emergency, and amorous ambush, Kimihito remains a steadfast, kind-hearted figure who accepts them not as monsters, but as equals. The overarching tension never fully resolves into a single romantic pairing, instead maintaining a chaotic, harem-style equilibrium where Kimihito continues his exhausting, bizarre, yet deeply heartwarming
everyday life with his ever-expanding family of lovable monster girls.