2年前,科幻小说家大卫·戈登(约翰·库萨克 John Cusack 饰)遭受了丧妻之痛,时间并未抚平他心中的伤口,反倒令其愈感空虚。为了排遣心中的孤独,大卫决定收养一个小孩,当然这个决定遭到周围亲友的质疑和反对。不顾他人的意见,大卫领养了名叫丹尼斯(鲍比·科尔曼 Bobby Coleman 饰)的小男孩。丹尼斯虽然清秀可爱,性格却非常古怪。他自称来自火星,终日躲在纸盒子里,不与其他小朋友玩耍,喜欢用没有胶卷的相机拍照。丹尼斯的到来为大卫的生活注入活力,也为他带来久违的爱情。而随着生活的继续,大卫却发现与火星男孩的相处实非易事…… 本片根据David Gerrold的获奖科幻短篇小说改编。What's the nature of being a parent and of being a child? David is a widower grieving for two years. He writes science fiction and was considered weird as a boy. He meets Dennis, a foster child who claims to be on a mission from Mars, stays in a large box all day, fears sunlight, and wears a belt of flashlight batteries so he won't float away. David takes the six-year-old home on a trial. His sister and his wife's best friend offer support, but the guys are basically alone to figure this out. Dennis takes things, is expelled, and is coached by David in being normal. Will the court approve the adoption, and will Dennis stay? Can a man become a father and a child become a son?