"Rakuen Danshi - Beast Harem" centers around the life of Itsuki Amakawa, a high school student who unexpectedly inherits a mysterious mansion from his late grandfather. Upon moving into the estate, Itsuki discovers that the mansion is inhabited by a group of anthropomorphic animal girls, each representing a different species. These girls, known as "Beast Girls," are bound to the mansion by a magical contract and serve Itsuki as his loyal companions. The narrative explores the dynamics between Itsuki and the Beast Girls as they navigate their daily lives together, blending elements of comedy, romance, and supernatural intrigue.

The Beast Girls each possess unique traits and personalities tied to their animal origins. For example, Neko, a cat girl, is playful and mischievous, while Inu, a dog girl, is fiercely loyal and protective. Other characters include Usagi, a shy and gentle rabbit girl, and Tora, a bold and confident tiger girl. The interactions between Itsuki and the Beast Girls often lead to humorous and heartwarming situations, as they adapt to living together and forming bonds despite their differences.

As the story progresses, Itsuki learns more about the origins of the Beast Girls and the magical contract that binds them to the mansion. The narrative delves into the challenges they face, including external threats and internal conflicts, as they strive to coexist peacefully. Themes of trust, loyalty, and acceptance are woven into the plot, as Itsuki and the Beast Girls work together to overcome obstacles and strengthen their relationships.

The series balances lighthearted moments with occasional dramatic tension, creating a narrative that appeals to fans of harem and supernatural genres. The interplay between Itsuki and the diverse cast of Beast Girls drives the story forward, offering a mix of comedic escapades and emotional depth. The mansion itself serves as a central setting, with its mysterious history and magical properties adding an element of intrigue to the overarching plot.