"Maria Watches Over Us" is a slice-of-life anime series set in the fictional Lillian Girls' Academy, a prestigious Catholic school for girls in Tokyo. The story revolves around the unique "sœur" (sister) system, an upperclassman-underclassman mentorship tradition where older students guide younger ones. The narrative primarily follows Yumi Fukuzawa, a first-year student who becomes the petite sœur of Sachiko Ogasawara, a refined and elegant third-year student. Their relationship forms the emotional core of the series, exploring themes of friendship, loyalty, and personal growth.

The series begins with Yumi, an ordinary and somewhat shy girl, unexpectedly chosen by Sachiko to be her petite sœur. This decision surprises many, as Sachiko is known for her aloof demeanor and high social standing within the school. As their relationship develops, Yumi navigates the complexities of the sœur system, learning to balance her admiration for Sachiko with her own sense of self. The bond between the two deepens over time, with moments of tension and misunderstanding giving way to mutual respect and affection.

Supporting characters include Sei Satou, a charismatic and enigmatic third-year student who becomes a close friend and confidante to Yumi, and Shimako Toudou, a reserved and introspective girl who forms a sœur relationship with Noriko Nijo. These relationships further enrich the narrative, highlighting the diverse dynamics within the school. The series also delves into the lives of other students, providing a broader perspective on the traditions and social hierarchy of Lillian Girls' Academy.

The plot unfolds episodically, with each arc focusing on specific events or challenges faced by the characters. These include school festivals, cultural exchanges, and personal dilemmas, all of which serve to deepen the relationships and reveal the characters' inner struggles. The series maintains a gentle, contemplative tone, emphasizing emotional connections and the quiet moments of everyday life.

Throughout the series, the sœur system is portrayed as both a source of comfort and a potential burden, with characters grappling with the expectations and responsibilities it entails. The narrative avoids overt drama, instead opting for subtle character development and nuanced interactions. The result is a heartfelt exploration of adolescence, identity, and the enduring bonds of friendship.