Nuclear scientist Larsen, captured by the Chinese, is rescued by US secret services. However, during the escape, Agent Z 51 is killed, and Larsen's whereabouts become unknown. Agent Z 55 is then sent to Hong Kong to finish the mission. The task is complicated by the fact that Larsen is also being pursued by a criminal organization seeking to deliver him to the Chinese authorities for a reward. With the assistance of a Chinese girl named Su-Ling, Z 55 manages to successfully rescue the scientist.