Due to a weather problem a plane from Denmark is forced to land at the Slovene airport. Amongst the passengers, being taken to a hotel in Ljubljana, is a quiet, beautiful young Danish girl Iben (25). This is how she meets Tina (25) who drives a shuttle as a summer job. They both need each other, but are very careful because one of them is hiding a secret. They speak to each other in English and in their native languages. Only one of the two languages has the so-called dual, a special grammatical form used in expressions that involve just two subjects. The other language does not. Dual is an honest story about two people who meet precisely at the moment when they absolutely should not have - but they are nevertheless a bit happier because they did.
表面上，雪莉（珍妮弗·加纳 Jennifer Garner 饰）是一名普通的大学研究生，实际上，她已为名为SD6的秘密情报组织工作多年。在同男友结婚之际，雪莉决定向其公开自己的真实身份，没想到男友却因此被组织的安全人员暗杀。愤怒中，雪莉选择了“罢工”，之后亦遭到了抹杀。
危机之中，雪莉知道了自己的父亲杰克（维克托·加柏 Victor Garber 饰）也是和自己一样的秘密特工，并且，在背后扶持SD6的并非国家组织，而是拥有强大武力的反动分子。于私于公，雪莉和SD6都有着不共戴天之仇，她回到了SD6，但此时，雪莉的身份已升级成了为CIA工作的双重间谍，在危险和考验之中，雪莉的任务只有一个，那就是彻底铲除SD6。
