一个老油条警探尼克麦克坎（伯特·雷诺兹饰）一向是单打独斗、雷厉盛行。在一次办案中，不意一个八岁男孩狄凡巴特勒成为一件犯法案件的唯一证人，而男孩向尼克提出要求，除非让他当一天差人，并提供徽章、手拷等全数差人装备，他才愿意出头指证，尼克不得已准许，成果狄凡当差人当出了瘾，产生连串惊异冒险，尼克面临“小鬼差人”，头痛不已。

Devon Butler is an eight-year old who dreams of being a cop. He watches police TV shows, knows police procedures, and plays cops and robbers with his friend Ray. One day, while snooping around in a warehouse, he witnesses a murder. He goes to the police, who want the information, but won’t get it until they make Devon …