A Deadly Secret, also translated as Requiem of Ling Sing and Secret of the Linked Cities, is a wuxia novel by Jin Yong (Louis Cha). It was first published in the magazine Southeast Asia Weekly (東南亞周刊) and the Hong Kong newspaper Ming Pao in 1963. Its original Chinese title was Su Xin Jian before Jin Yong changed it to Lian Cheng Jue. The story revolves around the adventures of the protagonist Di Yun, an ordinary young peasant, who is imprisoned after being framed. In his quest for vengeance, he accidentally acquires the Liancheng Swordplay manual (連城劍谱), an ancient artefact not only prized for the skills detailed inside, but also for containing a secret leading to a treasure.
The plot follows the experiences of the protagonist Di Yun, a simple young peasant from Xiangxi. He lives in the countryside for several years together with his martial arts teacher, Qi Zhangfa, and Qi's daughter, Qi Fang, who is his childhood sweetheart. One day, the three of them travel to the city to attend the birthday party of Wan Zhenshan, Qi Zhangfa's senior from the same martial arts sect. Di Yun is framed for larceny and attempted rape, which results in him being arrested and imprisoned.
Qi Zhangfa disappears mysteriously when Di Yun needs his help. Wan Zhenshan's son, Wan Gui, bribes the magistrate to hand a heavy sentence to Di Yun to exaggerate the seriousness of Di's "crimes". At the same time, in order to win Qi Fang's affection, he hypocritically plays the role of a good man by pretending to help Di Yun. Qi Fang becomes disappointed with Di Yun after believing that he is indeed guilty and gives up on him. With no one else to turn to, she eventually marries Wan Gui.
Di Yun suffers in prison and is continuously harassed by Ding Dian, a fellow raving inmate who accuses him of being a spy and subjects him to constant beatings. However, after Di Yun attempts suicide, Ding Dian is convinced that he is not a spy and befriends him. Ding Dian tells Di Yun how he obtained from Mei Niansheng the manual for the skill "Liancheng Swordplay", and how he became the target of several martial artists after getting the manual. Ding Dian also teaches Di Yun a powerful inner energy skill, which later proves to be a blessing for Di. Di Yun also learns from Ding Dian about the dirty secrets of Qi Zhangfa and his fellows – of how they murdered their teacher, Mei Niansheng, to seize control of the Liancheng Swordplay manual.
Di Yun and Ding Dian manage to break out from the prison but Ding is fatally poisoned by Ling Tuisi, a heartless magistrate who is also the father of Ding's deceased lover. Di Yun returns to Qi Fang's house and sees that Qi has bore Wan Gui a daughter nicknamed "Kongxincai" – his childhood nickname. Feeling depressed and emotionally hurt, he leaves and arrives at a temple, where he encounters an evil cannibalistic monk, Baoxiang, whom he outwits and kills. He dons Baoxiang's robes and is mistaken by the Tibetan Blood Saber Sect's lascivious leader, Grandmaster Xuedao, as a grand-apprentice. Xuedao protects Di Yun from attacks by self-proclaimed orthodox martial arts sects, captures a maiden Shui Sheng, and holds her hostage while they flee from the attackers.
They encounter an avalanche that causes them to be trapped a snowy valley in the Daxue Mountains. Xuedao manages to kill three of their pursuers, one of whom is Shui Sheng's father. Meanwhile, Xuedao becomes suspicious of Di Yun's identity and attempts to kill him when Di's cover is blown. Unexpectedly, Xuedao's strike helps Di Yun complete his inner energy cycle; Di Yun turns the tables on Xuedao and kills him. The last surviving pursuer, Hua Tiegan, reveals his true colours after Xuedao's death and feeds on the dead bodies of his three companions to survive. While Di Yun, Shui Sheng and Hua Tiegan remain in the valley to wait for spring, Shui sees Di's kindness beneath his seeming misanthropy. When the three of them are finally able to leave the valley and meet up with other martial artists, Hua Tiegan accuses Di Yun and Shui Sheng of sexual immorality in front of Shui's fiancé. Di Yun separates himself from Shui Sheng and continues on his lonely quest for vengeance.
Di Yun tracks down the perpetrators responsible for his wrongful incarceration and learns that his respected teacher, Qi Zhangfa, is actually a scheming and ruthless villain – just like what Ding Dian had told him. Qi Fang is mercilessly killed by her husband, Wan Gui, when he suspects her of infidelity. As the story progresses, all the antagonists in the novel eventually locate the whereabouts of the Liancheng Swordplay manual in a temple, where they start fighting over the treasure. They become insane after coming into contact with the deadly venom smeared on the jewels.
After witnessing these beastly acts, especially Qi Fang's death, Di Yun becomes totally disillusioned with the dark nature and greed of humanity. He brings Qi Fang's daughter, Kongxincai, with him to the snowy valley and intends to lead a reclusive life there. To his surprise, he meets Shui Sheng, who has been faithfully waiting alone for his return.
The novel's historical setting is not explicitly mentioned. However, some readers speculate that the story is based in the late Qing dynasty, judging from the illustrations in the published Chinese versions authorised by Jin Yong, which show male characters wearing queues (a specific Qing era hairstyle).
Wu Liuqi, a character from The Deer and the Cauldron, is mentioned in the third edition of A Deadly Secret. This confirms that A Deadly Secret is set in the Qing dynasty.
Some commentators feel that the plot resembles Alexandre Dumas' The Count of Monte Cristo, except that they are based in different countries and historical periods. Jin Yong has admitted some influence from Dumas, his favourite non-Chinese novelist.
In 1981, Hong Kong's RTHK produced a 25 episodes radio drama based on the novel.