The Twelve Zodiacs.

Lieutenant Jeng's Police Unit was out to capture a drug dealer, Yai, who killed his best friend and colleague. Suspended from the mission, Jeng takes the mission into his own hands and goes undercover to capture Yai. But as Jeng gets hotheaded and rushes in, the plan is destroyed, the bad guys get away Jeng gets the police station sued for assault and attempted murder by Yai and also put on probation himself. Because of his probation period Jeng is put on a suicide case.