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.

这是一个甜美却酸涩的青春爱情故事。男孩在搬家前一天，邀请曾经青梅竹马的女孩一起随便走走，女孩欣然答应。于是两人踏上了前往海边放烟花的旅程。电车上，男孩提起儿时他们之间的一个秘密——共同偷了一枚手榴弹。两人相约找个地方把它埋掉。男孩和女孩，一路上刺激地骑着偷来的自行车，又跟暴走族斗智斗勇。终于来到了海边时，烟花却没能放成。两人相互表白，可这注定是一段无疾而终的爱情。 最终，火车还是轰鸣地带着男孩去了远方。突然间在黑暗中，出现了一个熟悉的身影，两串璀璨的烟火。