"Aitsu to Lullaby" is a manga series that explores themes of family, identity, and the complexities of human relationships. The narrative centers around a young man named Kouta, who unexpectedly becomes the guardian of a baby named Haru after the sudden death of Haru's mother, Kouta's estranged sister. Kouta, initially unprepared for the responsibilities of parenthood, struggles to balance his own life with the demands of caring for an infant. The story delves into his journey of growth as he learns to navigate the challenges of raising a child while confronting unresolved feelings about his fractured family.

Kouta's life takes a dramatic turn as he forms a bond with Haru, whose presence forces him to reevaluate his priorities and confront his past. The series portrays the daily struggles of single parenthood, including sleepless nights, financial pressures, and societal judgment, while also highlighting moments of tenderness and joy. Kouta's relationships with those around him, including his coworkers, friends, and Haru's potential father, add layers of complexity to the narrative. The story examines themes of responsibility, sacrifice, and the transformative power of love, as Kouta gradually evolves from a reluctant caregiver to a devoted father figure.

The characters are portrayed with depth and nuance, with Kouta's internal conflicts and growth serving as the emotional core of the series. Haru, though an infant, becomes a catalyst for change in Kouta's life, symbolizing hope and the possibility of redemption. The manga also explores the perspectives of secondary characters, such as Kouta's sister's former partner and Kouta's supportive friends, who provide insight into the broader context of the story. Through its exploration of familial bonds and personal growth, "Aitsu to Lullaby" offers a poignant and heartfelt portrayal of the challenges and rewards of parenthood.