Mama is a 4th Grade Student

Because her father was sent to London to work, Natsumi, a 4th grade student, also must leave Japan along with him. Her classmates throw a farewell party for her. Natsumi is prepared to go to London, but on the night before Natsumi's arranged departure, a baby suddenly appears in her house after a roar of thunder! What's more is that this little baby is actually the one that Natsumi will give birth to in fifteen years!