"NAKAIMO - My Little Sister Is Among Them!" is a romantic comedy anime series centered around Kōta Shikita, a high school student who lives alone after his parents' divorce. His life takes an unexpected turn when his father, a wealthy businessman, offers him a substantial inheritance on the condition that he identifies and lives with one of his six potential half-sisters, all of whom he has never met. These sisters, unaware of their shared father, are introduced into Kōta's life under the guise of being transfer students or new acquaintances. The series explores the comedic and dramatic situations that arise as Kōta navigates his relationships with these sisters while trying to uncover which one is his true sibling.

The six sisters each have distinct personalities and backgrounds, adding variety to the narrative. They include Mizuki, a cheerful and athletic girl; Mana, a shy and reserved bookworm; Rinka, a confident and assertive model; Mikazuki, a quiet and mysterious transfer student; Hina, a bubbly and energetic idol; and Sora, a mature and responsible older sister figure. As Kōta interacts with them, he begins to form bonds, complicating his initial goal of simply identifying his sister for financial gain. The series balances lighthearted humor with moments of emotional depth, particularly as Kōta grapples with his growing feelings for the sisters and the moral implications of his father's challenge.

The plot unfolds through a series of episodic encounters, with each sister receiving focus to develop her character and relationship with Kōta. The narrative occasionally delves into themes of family, identity, and the complexities of human connections, though it primarily maintains a comedic and romantic tone. The series concludes with Kōta making a decision about his inheritance and his relationships, though the resolution leaves some aspects open to interpretation. "NAKAIMO - My Little Sister Is Among Them!" is characterized by its blend of harem elements, sibling dynamics, and situational comedy, offering a mix of entertainment and light drama.