The series follows the exploits of Ryo Saeba, a "sweeper" who is always found chasing beautiful girls and a private detective who works to rid Tokyo of crime, along with his associate or partner, Hideyuki Makimura. Their "City Hunter" business is an underground jack-of-all-trades operation, contacted by writing the letters "XYZ" on a blackboard at Shinjuku Station.
One day, Hideyuki is murdered, and Ryo must take care of Hideyuki's sister, Kaori, a tomboy who becomes his new partner in the process. However, Kaori is very susceptible and jealous, often hitting Ryo with a giant hammer when he does something perverted. The story also follows the behind-the-scenes romance between Ryo and Kaori and the way they cooperate throughout each mission.