The film tells a story of 2 girls who were born on the same day and unintentionally switched at birth. Their lives are completed changed due to the misplacement when they are returned to their respective families.
Tensions arise when Jack Chan, a spineless constable who regards his work as a job but not a career, is partnered with an energetic and conscientious policewoman, Man Liu. When a serial killer emerges in Hong Kong, Man Liu swears to catch him. Jack is impressed by her bravery and decides to secretly protect her.