The story commences with an interview, as the interviewees piece together the legendary life of Rong Jinzhen through recollections. Turning back the clock to the early 20th century, during the chaos of warlords and political unrest, the twelve-year-old Rong Jinzhen displayed exceptional mathematical talent. At the age of sixteen, he was admitted to N University, where his mentor, Professor Hayes, recognized and cherished his abilities. Their relationship evolved into a deep friendship. Upon discovering Rong Jinzhen's extraordinary talent in code-breaking, Hayes cautioned him against becoming overly fixated on "decode." With the outbreak of war, the two sympathetic individuals had to choose vastly different paths. Hayes returned to his homeland, responsible for "creating" codes, while Rong Jinzhen joined the 701 Unit, specializing in "deciphering" codes.
At that time, the looming concern for everyone at 701 was the enemy's "Purple Code." Some even went mad due to the inability to find a solution. In less than a year, Rong Jinzhen successfully cracked the "Purple Code." However, faced with ensuing love, he chose to delve deeper into the path of "decode." Through correspondence with Hayes, Rong Jinzhen gradually realized that his former mentor had become his adversary. The destinies of the two ultimately intertwined, and the final showdown between master and disciple would unfold around a code more advanced, sophisticated, and weighty than the "Purple Code" – the elusive "Black Code."