"Kyō kara Aniki desu!!" centers on the lives of two brothers, Shōta and Ryōma, whose relationship undergoes a significant transformation after a sudden and unexpected event. Shōta, the younger brother, is a high school student who has always admired his older brother Ryōma, a charismatic and confident individual. However, their dynamic shifts when Ryōma, who had been living independently, suddenly returns home and declares that he will now act as Shōta's "aniki" (big brother) in a more deliberate and overbearing manner.

The narrative explores the evolving relationship between the two brothers as Ryōma imposes his presence on Shōta's daily life, often in exaggerated and comedic ways. Ryōma's actions range from offering unsolicited advice and guidance to inserting himself into Shōta's school and social activities, much to Shōta's initial annoyance and embarrassment. Despite his resistance, Shōta gradually begins to appreciate Ryōma's efforts, recognizing the underlying care and affection behind his brother's eccentric behavior.

The story delves into themes of familial bonds, personal growth, and the complexities of sibling relationships. Through a series of humorous and heartfelt interactions, the brothers navigate misunderstandings, challenges, and moments of genuine connection. Supporting characters, including Shōta's classmates and friends, add depth to the narrative, often serving as foils or catalysts for the brothers' evolving relationship.

"Kyō kara Aniki desu!!" balances comedy with emotional undertones, presenting a lighthearted yet meaningful exploration of brotherhood. The series captures the nuances of sibling dynamics, blending exaggerated humor with moments of sincerity to create a narrative that resonates with readers familiar with the joys and struggles of familial relationships.