Lee Gun (Jang Hyuk) is the successor to a large company run by his family. Kim Mi Young (Jang Nara) is an average-looking secretary in a law firm who earned the nickname “Post-It” for her inability to say no to her colleagues who dump work on her. When the two cross paths one night after separately having drunk a drugged liquid a comedy of errors occurs. Kim Mi Young goes into the wrong hotel room because the hotel door number accidentally changes, and the door is propped open by the mischief-making drugged liquid makers. She ends up sleeping with Lee Gun. When Mi Young finds out afterwards that she is pregnant, what will become of them? And will Kang Se Ra (Wang Ji-won) and Daniel (Choi Jin Hyuk) just stand by and let Gun and Mi Young deal with the situation in their own way?
The wealthy heir to a large hair product company with a trademark maniacal laugh. He is a descendant, the only one of his generation, of the distinguished Jeonju Lee clan, a lineage that has all the males dying in their thirties due to Huntington's disease. Though utterly devoted and affectionate towards the people he cares about, Lee Gun can be cold, selfish, and prickly to everyone else. However, he is genuinely warm-hearted and responsible, wanting to do his best for Mi-young and their baby. Although he was shocked and reluctant at the news of a shotgun wedding, Gun starts warming up to Mi-young's sweet nature and pure heart.
A naïve office worker at a law firm. She has no wealthy family, prestigious education, or much beauty, but rather comes from a small island where her mom owns a small restaurant. Because of her timid and kind-to-a-fault nature, she cannot bear to turn down a request from anyone and is nicknamed the "post-it girl," someone who is necessary but unimportant. While she lacks self-confidence, Mi-young always sees the best in people and upholds a quiet strength. When an accidental one-night stand gets her pregnant, her plain and insignificant existence changes as Lee Gun enters her life.
An internationally famous Korean-American designer whose single goal is finding his long-lost biological sister. He meets Mi-young by accident, and is intrigued by her kindness. From then on, he becomes her Daddy-Long-Legs, encouraging her when she is down and coming to her rescue when she needs help. Constantly worrying about her, Daniel begins to realize that what he feels towards Mi-young is something more than protectiveness. He is the biological brother of Kang Se-ra.
The love of Lee Gun's life. She is a ballerina in New York City with a surprisingly no-nonsense attitude. She has been with Gun for 6 years, delaying any plans of marriage for her career. But when she fails to get a lead part, Se-ra comes back to Korea to stay with Gun. A sudden callback has her choosing ballet over Gun one more time, but when she comes back for good, she finds drastic changes in Gun's life. She is the long-lost biological sister of Daniel Pitt.
Due to its popularity, the program is also broadcast in selected countries worldwide. Due to Jang Na-ra's popularity in China, the online broadcasting rights went for US$120,000 (₩120 million) per episode, making it the most expensive Korean drama sold in China at the time. It broke previous records held by Three Days and Doctor Stranger, which received US$50,000 and US$80,000 per episode, respectively. This record was later surpassed by My Lovely Girl at US$200,000 per episode. In the Philippines, it aired on ABS-CBN from January 26, 2015 to May 1, 2015 every weekday afternoons at 4:00pm under the network's Kapamilya Gold block. The series re-aired on Jeepney TV from September 19 to October 20, 2016.