With Seoul's crime rate on the rise, the local police are doing everything in their power to keep their citizens safe. Special Investigation Unit led by the honored graduate and discharged soldier of the Special Forces, Captain Lee Kangho is at the prime of his career, leading a group of hand picked individuals for the past two years at the Bureau. With a formidable reputation, his team is known throughout the districts for their work and solving rate. Senior Inspector Choi Beomgyu with his wise and fair judgment as well as twenty years of service, he looks after the younger team members. Detective Kim Minhyun with his fast wits and computer forensics background, Detective Nam Hyesoo with her attention to detail, the team is set. The SIU has another member, Zero. A hardworking, highly skilled and trained individual heavily involved in the underground scene. Since the unit began, Zero had the most arrest rates and assisted with many troublesome cases. The identity and operation of Zero is a secret to the Bureau and no one, but Captain Lee really knows when Zero came to be. The trust he had in his new founding team allowed Zero and SIU to work in harmony. The codename itself spread fear among the criminals and twisted into urban stories by the public. No one knows the reason for this partnership as Zero only works for Zero. Detective Park Jisung is a passionate new recruit who joins Captain Lee's team. Since he was young, his physical capabilities surpassed most of his peers and his strong sense of justice often landed him in trouble. In the Academy, he was admired by all and was established early on as a someone to look out for. With his own secret to joining the SIU, how will he adapt to his new environment and secretive member?