In 2071, roughly fifty years after an accident with a hyperspace gateway made the Earth almost uninhabitable, humanity has colonized most of the rocky planets and moons of the Solar System. Amid a rising crime rate, the Inter Solar System Police (ISSP) set up a legalized contract system, in which registered bounty hunters, also referred to as "Cowboys", chase criminals and bring them in alive in return for a reward.

Takao, who is training to become a shoemaker, skipped school and is sketching shoes in a Japanese-style garden. He meets a mysterious woman, Yukino, who is older than him. Then, without arranging the times, the two start to see each other again and again, but only on rainy days. They deepen their relationship and open up to each other. But the end of the rainy season soon approaches.

近未来的日本，一个叫做“疏濑”的城市中，市民过着平凡的生活，只不过，在城市的一角，山冈上面耸立着医疗机器制造厂建造的外形酷似熨斗的工厂，每天固定时刻，那个工厂都像刚烫完东西一样散发出大量蒸汽。
小学六年级学生直太，与高中女生真见美，直太那个留学的哥哥的女友，一起在河边打发时间。本该是继续平凡的日子，随着传说中的蜜蜂女——睛子驾驶的电车撞上直太而结束了。直太头上长出了奇怪的角！而睛子居然成为直太家的佣人进入了直太家。睛子的秘密与她口中常念起的“FLCL”的含义，令直太的生活有了翻天覆地的改变。