Nanami 18-sai Mama ni Narimashita

Nanami is an 18-year-old boy whose older sister has run away from a member of the Japanese mafia. In her absence, Nanami is forced to take her place as the caretaker of the sister's baby and becomes a captive himself. He must navigate the challenges of being a young, unwilling mother figure while living under the watchful eye of the dangerous man his sister fled from.

As Nanami struggles to adapt to his new role, he faces the constant threat of the mafia member's unpredictable behavior. The baby's needs and his own survival become intertwined, forcing him to find a way to protect both himself and the child in a tense and uncertain situation.