While on patrol duty, EU sergeant Ling Sun-fung and her team are called to assist a criminal chase. A gun battle erupts and Sun-fung loses her father, who dies from a stray bullet to the head. Sun-fung continues the pursuit, but a strange natural phenomena suddenly turns her away, taking her back to April 1—three days prior to the incident. Realising that she has traveled back in time, she uses this opportunity to save her father's life.
Village chief "Ditch", an informant for the ICAC, discovers Sun-fung's special ability and teams up with her to investigate a corrupted cop. Unfortunately, their persistent disruption to the timeline causes severe butterfly effects, with unsettling consequences. Sun-fung learns that her power to change the past does not make her able to control the future.