Lee Sun is Joseon's Crown Prince. He, however, stumbles upon information about his father's history before acquiring his position as king. King Yeongjo is a product of a corrupt leadership contract. Things worsen for Lee Sun as his values towards equality clash with his father's ideology of taxation and a strong royalty. Both sides hold on to a double-edged sword, but only one will survives the wounds incurred by the Secret Door.

Four rookie detectives come from different backgrounds, dreams, and past lives. They bump into each other on the most cherished Gangnam Police Department, which isn't exactly the dream station they hope to find for their career. Dae Koo, Soo Sun, Tae Il, and Kook are led by their team leader Legendary Detective Pan Seok. What Pan Seok doesn't know is that he's dealing with the son of a victim from his one and only mismanaged case. Team 3's out to serve and protect Gangnam's justice system.

Kim Woo Hyun is the only son of a high-ranking police officer. He breezes through the police academy and graduates from it with honors. After being assigned to the cyber investigation department, Detective Kim finds himself entrenched in a cat and mouse game with a faceless enemy in the cyber world. Tirelessly hunting down the hacker, alias of Hades, he finds out that Hades is his past roommate in the police academy, Ki Young. Ki young is caught. But he escapes and tries to find an evidence proving his innocence. Meanwhile, Woo Hyun tries to reveal the secrets of those that hide within the cyber world.

Her name is Gong Ah Jung. She's very nice but has never thought about getting married. She thought she would be free forever, but one day, she realized being single is cumbersome. She laughed at her friends who would get old under the name of a madam, but now she is worried that she will become a person worse than that title. She studied and worked hard and ran hard without fear, but she was pushed to the lowest rank in all kinds of social benefits in the community. What does it mean to get married? She dreams of love but always shouts, "Love is so much." Can she succeed in finding her true love?

Gang Jin, who moved around frequently with her mother, now works as a bartender to get by while she studies to be the top student in school. Living a rough life and raised with strong values, Gang Jin finds her only hope of love in Ji Wan at the end of his teen years. Yet when Ji Wan loses her older brother when he tries to find Gang Jin's pendant, she leaves her hometown, and they separate. It is not until ten years later that Gang Jin and Ji Wan reunite.

Eun Sung is a charming girl with a pure heart. She is easy-going and down-to-earth, which helps her to view the world in a positive light. One day, Eun Sung's father loses a lot of money to her stepmother. To make matters worse, she even loses her brother, who is a young, autistic boy. When she has to live on the streets amidst hardship, she runs into an old lady with dementia and helps her. And she happens to be the owner of a company. Eun Sung gets to live in the old lady's big house, but her life there doesn't go well because of others' jealousy. We invite you to the story of a girl who overcomes obstacles with hope, courage, and love.

At 8-years-old, Yong witnesses his father's murder. This leads to Soe Dol adopting Yong and marks the beginning of Yong's life as a ruffian. Unlike most noble women, Eun Chae is smart and shrewd. She envisions the concept of an inn and goes into business with her father to establish Joseon's first-ever upscale inn. While doing so, she finds herself falling for Yong, who keeps himself hidden behind his black mask. Yong realizes that there is more than meets the eye regarding his father's death and decides to dig deep to investigate the matter. While searching for a special design pattern etched on the sword that took his father's life, Il Ji Mae begins his quest of taking revenge against evildoers.

Han Eok Kwan, an orphan and a gangster, lives off of gambling and taking money from merchants. One day, he takes in Jae Soo, the son of a woman he had lived with in the past, and becomes a father. When Eok Kwan is about to give up his life, he meets Hye Rim, the mother of Soo Ah and Kyung Ho. Eok Kwan and Hye Rim marry, but she dies shortly after. Soo Ah grows to resent Eok Kwan after her mother's death. Kyung Ho has also had a troubled youth. Jae Soo ends up in prison for his love for Soo Ah and his guilt over Kyung Ho. Eok Kwan lives with them as their father in a twisted family relationship.