Juliane Thomas is an ambitious but unemployed young writer. After breaking up with her lover she works at a dentist friend to make ends meet. One day she instantly falls in love with one of the patients (Jean Berner) and promptly writes a movie script about the encounter in which she projects her own fantasies about how things will turn out eventually. By coincidence this movie script is picked up by a film director who happens to be Berner's closest friend and from then on things become very complicated...

影片描绘的是登陆诺曼底的第一天所发生的事件，所谓最长的一天是该片分别从盟军和德军双方面的角度同时描述诺曼第登陆这一历史战役的来龙去脉，以及双方在此期间的重要反应，手法平实，场面逼真，让整个事件在观众眼前重现。并且粗中有细，无论是决策的将军还是登陆的士兵都写得栩栩如生：英军战机低空飞过攻击；美国特种部队遭德军射击；一群法国修女冒着枪林弹雨抢救联军伤兵……备战实况规模之伟大，叫人印象深刻；过程中做出的种种错误决定及各项导致这场历史上其中一场最浩大战争的大小事件，影响深远。