After ten years behind the bars in Taiwan, Kong Sai-Hao finally returns to Hong Kong. Besides hoping to find his daughter Yau-Yau to mend their relationship, he also plans for revenge. On one hand, he pretends to surrender to triad boss To Yik-Tin, the man who caused him to be jailed. On the other hand, he tries to gain trust from police officers Chung Lap-Man and Lee Pak-Kiu. Pak-Kiu refuses to have a criminal as his friend, but Lap-Man becomes a undercover cop in Sai-Hao's gang. Along with Sai-Hao, they overrule Yik-Tin and take over the mafia, and the two develop a misconceived mentor-student relationship. Unfortunately, Lap-Man gets fired from the police force before he can finish his mission in the mafia. He becomes Sai-Hao's people and turns his back against the police force.

本剧是学警系列的第三部曲，学警经训练及在机动部队工作後，将参与警队的实际工作。让观众一起经历警员们的成长过程，由受训到成为一名维持社会治安的警员，当中经历多少辛酸、挣扎及矛盾，为观众一一道来。

社團中人江世孝從台灣服刑回港，四處打聽愛女江悠悠下落之餘，不忘部署復仇。世孝向當年害他入獄的龍頭大哥杜亦天假意投誠，又誘導警員鍾立文及李柏翹成為自己人。柏翹堅拒與賊為友，但立文卻深信可將世孝轉為警方線人，二人逐步建立兵賊錯位的師徒關係。亦天對世孝常存戒心，安插親信梁笑棠挾制其勢力，世孝伺機利用亦天之妻程若芯在社團的影響力登上龍頭，販毒生意卻連遭滑鐵盧，始發現最信賴的身邊人比他更懂背叛、欺騙、出賣。