A Deadly Secret (Chinese: 連城訣; literal meaning Mnemonic of the Linked Cities) is a 1980 Hong Kong film directed by Mou Tun-fei, produced by the Shaw Brothers Studio, and starring Ng Yuen-chun, Liu Lai-ling, Jason Pai Piao, Shih Szu, and Elliot Ngok.

A student of the renowned martial artist Qi Zhangfa (Tong Kam-tong), Di Yun (Ng Yuen-chun), is sentenced to death on charges of larceny and rape.

He proclaims his innocence, claiming that he had been set up but is ignored by the sadistic jailers who regularly taunt and abuse him, and is horrified in their treatment of another inmate, Ding Dian (Jason Pai Piao), whom they and the presiding magistrate, Ling Tuisi (Elliot Ngok) seem to torture purely out of pleasure.

Di Yun, impressed at his martial arts prowess, implores Ding teach him to which the latter agrees and the young man proves to be an adept learner, making rapid progress.

Ling reveals he coated Shuanghua's coffin lid with a potent poison and offers the antidote in exchange for the manual, a lie Ding quickly discerns by pointing out the flower used to make it has no cure.

Ding tries to relay the manual's contents to Di Yun, but the latter refuses to listen and runs away where he happens to chance upon his master as well as Yan Daping and Wan Zhenshan, who try to kill him.

As Di Yun buries him with Shuanghua, he is attacked by Ling and in their fight, knocks over her coffin lid where the location of the treasure, pointed toward the Buddha statue in the Tian Ning Temple, is written there in her blood.