Cánh Cửa Bí Mật kể về Thirty years ago, hardened criminal Yip Siu-tin is arrested for armed robbery and sentenced to prison. Siu-tin’s daughter, who has handled the loot, changes her name to Cheung Sum-yuet and starts a new life. She also gets married to yacht company boss Yen Ho-yin. Meanwhile, Chong Chi-kiu is a kid left behind by an accomplice who has died because of the robbery. Kind-hearted Tang Sing-him becomes his foster parent. A traffic accident causes Chi-kiu and Sum-yuet to run into each other again. But this accident somehow has something to do with Siu-tin, who is just released from prison. Sum-yuet can no longer avoid Siu-tin, who is a nightmare to her. Her marriage also has certain issues. Police officer Yeung Yan-hiu intervenes by launching an investigation, and she keeps going after Siu-tin and Chi-kiu. Unresolved grudges for thirty years are again put on the front burner. Everyone implicated in a robbery of yesteryear is drawn into this mess.
