"I Don't Understand What My Sister's Friend Is Thinking" centers on the interactions between a high school student, his younger sister, and her enigmatic friend. The protagonist, a typical high school boy, finds himself increasingly puzzled by the behavior of his sister’s close friend, a girl who frequently visits their home. Her actions and expressions are often cryptic, leaving him unsure of her intentions or feelings. Despite her reserved demeanor, she occasionally displays moments of warmth and playfulness, further complicating his understanding of her.

The narrative unfolds through a series of slice-of-life scenarios, focusing on the protagonist’s attempts to decipher the girl’s thoughts and motivations. His sister serves as a bridge between the two, often facilitating their interactions while remaining oblivious to her brother’s growing curiosity. The story explores themes of miscommunication, subtle emotional cues, and the complexities of human relationships, all framed within the context of everyday life.

The main characters include the protagonist, a relatable and somewhat clueless high schooler; his younger sister, who is cheerful and straightforward; and her friend, whose quiet and mysterious nature drives much of the story’s intrigue. The dynamics between these characters are central to the narrative, with humor and lighthearted moments balancing the underlying tension of the protagonist’s confusion. The manga’s episodic structure allows for gradual character development while maintaining a focus on the protagonist’s ongoing struggle to understand the girl’s true feelings.