- The Emergency Unit (E.U.), capable of rapid response to any type of emergency, is primarily tasked to provide a speedy and additional presence of uniformed police on the ground to combat crime. Newly appointed sergeant LING SUN-FUNG (Tracy Chu), who is fully aware that they cannot afford any minor mistakes, often urges her team members to race against time. One day, while carrying out a patrol with her team, SUN-FUNG ends up in an exchange of fire with the criminals, during which she is repeatedly obstructed by the village chief, KWAN TING-MING (Vincent Wong). Due to that, not only do the suspects manage to escape, but SUN-FUNG is also forced to watch her father being killed by stray bullets.
Out of the blue, SUN-FUNG is able to travel back through time to April 1st on April 3rd and relive the same days over and over again, so she uses the time loop to her advantage and saves her father, without knowing that the altered timeline creates ripple effects. Having unwittingly learned about SUN-FUNG's secret, TING-MING seizes the chance to team up with her in a bid to track down corrupt cops, and subsequently the two become friends. However, TING-MING has been concealing his real identity. Meanwhile, those close to SUN-FUNG such as her co-worker/boyfriend KWOK SHEUNG-CHING (Benjamin Yuen), aloof driver CHAN KING-WAI (Paul Sinn) and her good friend/journalist LI CHI-TING (Zoie Tam) also find themselves involved in ensuing paranormal phenomena beyond their understanding or control...