In 2010, Cheuk Hoi secretly travels to Bangkok to conduct a drug operation with his undercover agents in Thailand. However the success of the mission had cost all of Cheuk Hoi's undercover agents to be killed in a sea of fire. Meanwhile back in Hong Kong, his good friend CIB Inspector Lai, is killed for unknown reasons, leaving Cheuk Hoi to take charge of a female amateur cop who is not yet officially an undercover. Cheuk Hoi knows that Lai Sir still has undercover agents whose identities have been deleted. To learn the truth about Lai Sir's death, Cheuk Hoi must work with Cheng Suk Mooi to uncover and secure the remaining of his late-friend's undercover agents while dealing with corrupt officials in the Police Force.

刑事情报高级督察卓凯（苗侨伟 饰）现身泰国曼谷，在当地获得重要情报：九指强与泰国倪坤进行毒品交易。追查之后发现吉运帮黑吃黑将倪坤杀死与九指强进行交易。为寻找吉运帮犯罪证据卓凯与卧底潜入社团窝点后被Pak Key得力手下乐少锋（周柏豪 饰）发现行动，和他们交起手来。逃脱之后卓凯一行人准备离开泰国返回香港时发生一场大爆炸，除了卓凯之外的所有卧底葬身火海。伤心欲绝的卓凯返回香港后面临停职，变得一蹶不振。与此同时香港最大的社团长兴发生内斗，长兴新继任的龙头魏德信（陈豪 饰）以雷厉风行手段剿灭社团的高级头目，其中包括覃欢喜（许绍雄 饰）。然而覃欢喜是长期潜伏在长兴的卧底，面临此番处境，他曾想过向Handler求助调回警察队伍，但妻子突然出事惨死在社团人士手中，令覃欢喜彻底沦入黑道。

《使徒行者2》（英文：Line Walker : The Prelude）係香港電視廣播有限公司製作嘅電視劇，由苗僑偉、陳豪、宣萱、袁偉豪、周柏豪同黃翠如演出，係《使徒行者》嘅前傳。套劇用警察臥底做主題。套劇由2017年9月18號到10月27號逢禮拜一到五晚上20:00喺騰訊視頻播映普通話配音及粵語原聲版本（每日更新一集，VIP用戶提前睇到下周五更新，PC端同埋平板端用戶可提前一個鐘收睇）；同日晚上21:30－22:30喺翡翠台播映粵語原聲版。