在这部妈妈是小学四年生喜剧/动画片中,水木夏美（兴梠里美 配音）是一个小学四年级的学生，她最大的心愿就是能够成为一个合格的母亲，好好照顾自己的孩子。也许是神灵听到了她的愿望，一天，一个模样可爱的婴孩来到了她的眼前。原来，这个叫做美奈（吉田理保子 配音）的孩子是夏美十五年后的女儿，惊喜的夏美决定担负起艰巨的重任，好好抚养美奈。
美奈的到来为夏美带来了不少快乐，却也制造了不小的麻烦。不过，这都算不了什么，最最让夏美感到头疼的是，美奈的父亲貌似并不是自己暗恋的深泽龙一（筱原爱实 配音），而居然是最最讨厌的山口大介（高乃丽 配音）！这可怎么办才好呢？在这十五年里，夏美和大介之间究竟发生了怎样的情缘？美奈又为何会来到夏美的身边？这一切谜题都有待解开。
Natsumi Mizuki is a 4th grader whose parents have gone to London. Her Aunt Izumi comes to live with her. Izumi is an aspiring manga writer who is trying to get her first big break, so she doesn't have time for a baby that appears out of thin air on Natsumi's first night alone. The baby's name is Mirai and her mother is *Natsumi*, but 15 years older. Somehow, Mirai had traveled back in time. Now the young Natsumi has to raise Mirai while trying to keep her 4th-grade life from falling apart and keep Mirai-chan secret from everyone. She has the dubious help of Izumi and some baby-care gadgets from the future.
本片真正的灵魂人物——小泉阿姨。寶貝甜心,跟這小四年生。小妈妈。小学一年纪的回忆。
