影片第一幅静止画面是：赤身裸体的克里斯蒂娜侧卧着熟睡，身后的保罗坐在床边吸烟。此“片花镜头”节选自剧情（自身独立），然后画面打出了“21克”的片名，电影开始。 这是一个关于生命的传递和延续的故事。陌路相逢的人们，偶然却又心甘情愿的传递着灵魂的交接棒。一次交通意外，克里斯蒂娜的丈夫丧命于杰克的车下，却为心脏衰竭的保罗提供了生存的可能。保罗把枪口对准自己的胸口，是对忏悔中的杰克的安抚和对恩人（克里斯蒂娜）的挽救（不然杰克将死于克里斯蒂娜之手）。影片以克里斯蒂娜怀上保罗的孩子告结，展示了一个生命火种的交易和轮回。
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