"Mitsuba-kun wa Aniyome-san to" centers on Mitsuba, a high school student who finds himself in an unconventional living situation after his older brother marries a woman named Yuki. Yuki, now Mitsuba's sister-in-law, moves into their household, bringing a dynamic shift to Mitsuba's daily life. The narrative explores the evolving relationship between Mitsuba and Yuki, as they navigate the complexities of their new familial bond.

Yuki is portrayed as a kind and caring individual, often going out of her way to support Mitsuba, whether through preparing meals or offering advice. However, her affectionate nature sometimes blurs the lines between familial and romantic feelings, creating moments of tension and confusion for Mitsuba. Despite these challenges, Yuki remains a stabilizing presence in Mitsuba's life, helping him grow and mature.

Mitsuba, on the other hand, is initially depicted as a typical teenager, somewhat awkward and unsure of how to interact with Yuki. Over time, he begins to appreciate her efforts and develops a deeper understanding of her role in his life. The story delves into Mitsuba's internal struggles as he grapples with his feelings, balancing respect for Yuki as his sister-in-law with the emotional complexities that arise from their close proximity.

The manga also introduces secondary characters, such as Mitsuba's classmates and friends, who provide additional perspectives and contribute to the narrative's progression. These characters often serve as foils to Mitsuba, highlighting his growth and the impact of his relationship with Yuki.

Themes of family, responsibility, and personal growth are central to the story. The interactions between Mitsuba and Yuki are portrayed with a mix of humor, warmth, and occasional drama, reflecting the nuances of their relationship. The narrative avoids overt sensationalism, instead focusing on the subtle emotional shifts that occur as Mitsuba and Yuki adapt to their new roles within the family.

Overall, "Mitsuba-kun wa Aniyome-san to" presents a thoughtful exploration of familial bonds and the challenges that come with navigating complex relationships. The story maintains a grounded tone, emphasizing character development and the everyday moments that shape Mitsuba and Yuki's connection.