Who's in the cast:
Lee Min-ho as Lee Gon
Kim Go-eun as Jung Tae-eul/Luna
Woo Do-hwan as Jo Eun-sup/Jo Yeong
Kim Kyung-nam as Kang Shin-jae
Jung Eun-chae as Koo Seo-ryung
There was a national treasure called Manpasikjeok, a bamboo flute named by King Sinmun during his reign in the Silla Dynasty. When the flute is played, enemies would retreat, diseases would be cured, rain would fall during droughts, rainy seasons would end, the wind would stop, and strong waves would subside. The Manpasikjeok is revealed only once every 20 years to wish peace upon the world.
The story starts in the Kingdom of Corea in the winter of 1994. King Lee-ho was betrayed by some of his guards that chose to support his illegitimate half brother, Prince Imperial Geum Lee-lim (Lee Gon's uncle). They stormed the King's palace in Cheonjongo and shot all the guards standing in their way. Prince Lee-lim approaches his younger brother and stabs him with the Four Tigers sword. Walking past the King's dying body, he gets hold of the flute.
Just then, a young boy enters the chambers with his bare feet covered in blood. He sees his father lying on the floor and runs over to him crying. He searches for the wound to try to put pressure on it and he calls for his father to wake up. His uncle tells him it's too bad that the Crown Prince is an orphan now. The boy struggles using all of his power to pick up the heavy sword. He manages to swing the sword downward and slice the flute in half. The Crown Prince tries to order the Royal Guards to arrest the traitor Lee-lim and deprive Prince Imperial Geum of his royal status for treason. All the men in the room were traitors and no one was there to follow his orders.
Lee-lim chokes the Crown Prince and presses the broken flute against his throat. He tells the boy that he's waited for this moment his whole life so how dare a useless kid get in his way. He shares that he quietly endured his father and brother's foolishness while gathering people over the years to assist him in his plan. Suddenly, the glass ceiling shatters and a masked shooter appears. The shooter takes out a majority of the traitors and during the commotion, Lee-lim drops one of the two flute pieces. With the Royal Guards on their way, he takes off leaving it behind. The masked shooter walks up to the Crown Prince to check his pulse before disappearing. The Crown Prince managed to hide one of the flute pieces under himself and grabs the masked shooters badge before blacking out.
The next day the country goes into mourning for the Royal Family as word gets out that Prince Imperial Geum is wanted for murder and treason after killing his brother King Lee-ho. The Crown Prince Lee Gon gets crowned as the new King since he's the first in line to the throne.
Lee-lim, now stripped of his title and in hiding, heads to the forest with his half of the flute. A large bright portal opens up to a parallel universe. Before he knows it, he's in the Republic of Korea in the winter of 1994. He looks the same, wearing the same clothes, but all the news articles don't match his world. Someone bumps into him and they immediately recognize each other. The man looks just like his dead half brother King Lee-ho, but in normal clothes. Lee-lim goes to meet the other version of himself and sees that he's a paraplegic in this universe. He ends up killing his brother again, this version of himself, and his nephew in this world.
During this time in Corea, the young King is forced to go through formalities as he mourns his father's death in the public eye. There's a little goofy boy that constantly tries to befriend the new King. The palace staff has a plan to let them constantly come in contact so King Lee Gon will have someone close. When the King is studying the little boy would enter the room and bother him with questions. The King would ask him how he got in and tell him to get out. The boy answers that their fathers were friends.
On the final night of the King's first duty of mourning, he finally shows some emotion by crying and claiming he misses his father. The little boy sees him crying and joins which makes him stop. The King tells the boy to stop crying and gives him the title Unbreakable Sword. He asks for the boy's real name which is Jo Yeong and from that day onward they were inseparable.
In the Royal Hospital in Corea, they discovered Lee-lim's body by the shore in what seems like a suicide incident. To keep things in order, they make a false statement to protect the young King and Royal Court by saying Lee-lim was killed by the Royal Guards. Without knowing it, the body they discovered was the dead paraplegic Lee-lim from the Republic of Korea universe.
We jump to the present-day Kingdom of Corea, Busan Main Palace in the Fall of 2019. King Lee Gon is now a full-grown adult being bothered by everyone about marriage. Jo Yeong, The Unbreakable Sword, is the Captain of the Royal Guards. Ironically, King Lee Gon jokes around a lot now, while Jo Yeong is extremely serious.
While the King was taking part in the Rowing Cup with his Navy Academy 88th class, he spots a person running in a rabbit hoodie. He chases the rabbit running barefoot through the streets. Captain Jo catches up to him holding shoes and asking why he ran off by himself. He claims to have seen the White Rabbit with the clock referencing Alice in Wonderland. Captain Jo doesn't understand so the King tells him he needs some fairytales in his life.
Back at the palace, Captain Jo requests that the King wear a bulletproof vest at all times. The King jokingly tells him that he can just wear two, enough for the both of them. Captain Jo states that the King is going to get him in trouble if he doesn't listen. The King asks him who's going to punish him if he's the captain. Captain Jo responds with his sense of duty. The King leans into him and smiles asking if he's having fun or would like to grab a glass of beer. Captain Jo deflects and wants to know why the King is so set on catching the culprit himself. The King explains that he's just trying to find his savior from 25 years ago. He doesn't understand why she just disappeared and never came forward.
King Lee Gon kept his saviors badge all these years. The badge belongs to Lieutenant Jeong Tae-eul from Seoul Metropolitan Police Station with her picture and birthdate. He was looking at it when he realized that now he's older than she was back then.
While feeding his white horse Maximus, the King spots the person in the rabbit hoodie again. He rides his horse and chases the rabbit alone. Captain Jo hops on a horse and tries to catch up to him. King Lee Gon notices something strange in the forest and Maximus starts to act up a bit. He sees the two pillars with the portal opening and decides to go through it. The other half of the flute that he carries around with him must have activated it.
In the Republic of Korea during the Fall of 2019, the Violent Crimes Division is overworked, underpaid, and catching all the bad guys. Lieutenant Jeong Tae-eul was in her car when she sees a white horse with a weirdo riding it walk past her. She uses the microphone to tell the man to park his horse immediately. All of the people around him are gushing over his handsome looks and assuming he must be filming a movie since he's on a horse. Lieutenant Jeong Tae-eul approaches the man on foot and as King Lee Gon turns to look at her, he can't help but stare. She tells him that he's causing traffic violations and asks him to get down from the horse. She realizes he's just staring at her and thinks it's because she forgot to show her badge. She puts her badge on as he gets off Maximus to walk towards her. He lifts her badge from around her neck to confirm her identity. He claims to have finally met her as he wraps his arms around her for a hug.