The Invisibles

Four HK police officers are in their intertwined careers and personal lives: Kan Yiu Yeung, Chan Yan Bo, San Si Lam, and Koo Ka Keung. The four of them were best of friends in their early days, working as officers in a counter-terrorism police organisation known as the CTRU (Counter Terrorism Response Unit).
Unknowingly, during one operation, a gunman stunned Nui Nui and was about to shoot him when Ocean made the decision to shoot Nui Nui’s leg and forcefully lower him enough to take out the gunman, saving Nui Nui’s life, but at a cost. While the tactic worked, Ocean miscalculated the severity of the injury he caused, and doctors had to amputate Nui Nui’s leg.
Although Nui Nui forgave Ocean, Yan Bo, Nui Nui’s little sister and Ocean's ex-wife, along with Ka Keung, never forgave him as both believed Ocean used Nui Nui as a pawn to help him rise up in his career. The incident would separate them and cause Ocean to suffer from PTSD and get kicked out of the CTRU. Ocean now works in a detective team called the POTT (Phantom Operation and Tactic Team) that solves high-danger level cases.
(Source: DramaWiki)