This paper is the first to comprehensively analyze the key legal controversies surrounding the Hong Kong National Security Law (NSL) and its implementation. Based on doctrinal analysis, case studies, and the most up-to-date statistics, this study centers on three categories of legal disputes: (1) the constitutionality and legality of the NSL; (2) the disputed content of the NSL; and (3) the legislative procedural issues involving the NSL. The study shows that the enactment of the NSL is not only an unprecedented crisis facing the “one country, two systems” framework, but also marks a culmination of the intersection and conflict between Chinese law and Hong Kong common law. The NSL has torn down the bulwark originally designed by the Hong Kong Basic Law and the Chinese Constitution to block infiltration of Chinese law into Hong Kong. A mixture of Chinese legal elements and Hong Kong laws, the NSL is a wedge inserted into the Hong Kong common law system. Through this law, many Chinese legal concepts, theories, and rules have inevitably been, and will continue to be, channeled into Hong Kong’s legal system. A “thin” version of “one country, two systems” may emerge in a post-NSL, or even post-2047, Hong Kong. The implementation of the NSL in Hong Kong also provides a valuable, if not the only, lens through which to observe whether and how a well-developed common law system can function under a heterogeneous authoritarian legal system and how the two might interact.