Set in a fictional present day where South Korea is governed under a constitutional monarchy, "The King 2hearts" tells the story of Prince Jae-Ha (Lee Seung-Gi) from South Korea and North Korean Special Force instructor Kim Hang-Ah (Ha Ji-Won) who become husband & wife through an arranged marriage.

....

In 1989, when the Berlin Wall collapses, a King and Crown Prince watch on TV, but the young Prince doesn't care about what is happening on the television. Prince Jae-Ha doesn't want even to become the king.

23 years later, Prince Jae-Ha takes military training. He isn't serious about anything, a playboy of sorts who looks forward to his discharge from the military. Meanwhile, the King (Lee Sung-Min) announces that military officers from South and North Korea will hold joint training sessions. The unified team will take part in the WOC (World Officers Contest).

Hang-Ah is a special forces instructor in North Korea. She is a great soldier with exceptional skills, but Hang-Ah longs to find a fine man to marry, just like other ordinary women. Due to her career in the military, she has never been in a romantic relationship. A high ranking military officer then asks Hang-Ah to join the WOC. Hang-Ah turns down the offer due to fears it might hurt her ability to eventually marry. The high ranking officer offers that if she joins the WOC, the North Korean government will take responsibility for her marriage and find her the right man. Hang-Ah accepts the offer and with two other members travels to South Korea.

Jae-Ha is the first one from the royal family who is discharged from the military as a low ranking private, but he doesn't care. Jae-Ha is just excited to get his release from the military. Unfortunately for Jae-Ha, he learns that he is selected as a member of the joint Korean team for the WOC. Jae-Ha is furious at his older brother, the King, but the King threatens to kick him out of the family if he refuses. Jae-Ha now has no choice but to join.

During the training period for WOC, North and South Korean military officers must room together. Jae-Ha sees his roommate, busy doing martial arts. Jae-Ha runs to a high ranking officer and asks for his own private room. The high ranking officer refuses his request and Jae-Ha must room with Hang-Ah.