饶承天（王喜 饰）是天才计算机犯罪专家，他喜欢挑战犯罪。而最近香港警察商罪科的高级督察庄展鹏（关礼杰 饰）则盯上了他，誓要将他入罪。一次，展鹏布置好一切因有承天犯罪，本以为天衣无缝一定能捉到他的把柄，岂料意外发生了一起枪击案，令一切努力都付诸东流，展鹏更是不服气。另一方面，警校毕业生家明邂逅了速递员健儿（胡杏儿 饰），而健儿则是承天同父异母的妹妹，只是两人还不知情。随后，家明在好心帮助同学进入警校资料库篡改成绩时被发现，遭到革职处理。展鹏则认为家明很有卧底的潜能，于是邀请他当卧底，将承天绳之于法。家明痛快的答应了。于是一场兵贼较量展开了。

Rao Chengtian (played by Wang Xi) is a genius computer crime expert who likes to challenge crime. Recently, Senior Inspector Zhuang Zhanpeng (Guan Lijie) of the Commercial Crimes Division of the Hong Kong Police has spotted him and vowed to convict him. Once, Zhan Peng arranged everything for committing a crime, and thought that he would be able to catch him in a seamless manner. Unexpectedly, there was an accidental shooting case, and all efforts were in vain. Zhan Peng was even more unconvinced. On the other hand, Jia Ming, a police academy graduate, met the courier Jianer (played by Myolie Wu), and Jianer is Chengtian’s half-sister, but they still don’t know. Later, Jia Ming was found when he kindly helped his classmates enter the police school database to falsify their grades and was dismissed.