In 1506, during the Joseon dynasty in Korea,
the tenth king Yeonsan was overthrown by opposing factions, making Jung Jong the new ruler.
However, those who helped him gain power rise up against him, causing anarchy in the nation. Amid
the chaos, a group of people is cruelly finished by guards who believe they have an unknown plague,
but one woman manages to save her little daughter before perishing. The girl suffers the loss of her
mother, but a guardian figure comes to her rescue when there are no more people around. At a temple,
the Prime Minister tells everyone present about the recent appearance of a mysterious creature
preying humans on Mount Inwangsan. The creature has been named ‘Monstrum,’ with royalty urged
to stop spreading rumors about its veracity. King Jung Jong addresses the Prime Minister, claiming
that he was the one who created the rumor to pressure the villagers and remove him from power.
The Minister advises him to reflect on his actions, as his wrongdoing and lowliness
created the Monstrum itself. Astonished by his confidence in speaking, the king talks about
finding someone trustworthy to investigate the rumors and prove them wrong. In the
forest, two clumsy hunters, brothers Sung Han and Yun-kyum argue about whether the beast is
real or not. Sung Han falls into his own trap, and then they are found by Yun-kyum’s daughter Myung,
who mocks her uncle’s ineptitude. The trio spends their days quietly hunting and fishing,
although they can barely scrape enough to feed themselves. Myung proposes going all the way to
Hanyang town, but Yun-kyum desists, thinking they are fine the way they are. Frustrated,
she goes out to work in the fields, but is soon surprised by a stranger who asks to see the
former commander of the royal guard, puzzling Myung. She aims her bow and arrow at the man,
claiming that no one with that description lives there. Just as Sung Han arrives and surprises her,
she accidentally shoots the arrow at the stranger, but he easily dodges it. By turning around,
Myung discovers a handsome young man with whom she falls in love. The boy introduces himself
as Officer Hur to Yun-kyum, revealing that he and Sung Han were once high-rank commanders.
Hur mentions the tremendous martial skills they used to had, but Myung can’t believe that story.
Suddenly, the king himself appears, leaving everyone perplexed. He and Yun-kyum talk
about their past, revealing that Yun-kyum was the soldier who picked up Myung as a child,
standing up against the king and disapproving his genocidal practice. The king’s followers
were confident that the girl possessed the creature’s plague and should be eradicated. Still,
Yun-kyum explains that the plague is transmitted through simple sight, but in the wake of him being
unaffected by the little girl, assumes it’s all fake, and innocent lives disappeared for nothing.
The guards take up arms against the man, branding him a traitor, but before attacking him, the king
stops the attack, and Sung Han appears to tell the king that his brother’s intentions aren’t bad.
Disappointed and saddened, the king spares their lives but orders them to leave the
place and never return. Yun-kyum offers his sword and leaves with his brother and the girl.
In the present, King Jung Jong tells the former commander that the creature’s rumor is getting
troublesome and offers him his sword in exchange for returning. After a night of thinking it over,
Yun-kyum agrees to go all the way to Hanyang, much to Myung’s excitement. Sung Han tries to
convince his brother to desist, but he already made his decision, and as his second-in-command,
Sung Han must accompany him. A cheerful Myung joins them on the journey, although
both are annoyed by this. In the forest, shamans appeal to the creature’s mercy with a ritual,
but a girl about to steal their food witnesses how the Monstrum kills them. Once in Hanyang,
Myung is disappointed by how sad and dreary the capital is. During lunch, the group is surprised
by Hur, as he calls them to see a crime scene in the forest. He shows the shamans’ remains,
with Myung realizing that there were signs of human combat at the scene. As the only witness,
they have the thieving girl, who says it was all caused by Monstrum. Yun-kyum is suspicious of her,
so he follows her along with Sung Han. The two split when the girl encounters a mysterious man.
While Sung Han loses track of the man, Yun-kyum goes after the girl, knowing that she lies in her
story. She says that, while the mysterious man was to blame for everything, the Monstrum is still a
real and dangerous threat, as no one dares to go to the mountain to search for resources anymore
for fear of being attacked. The group gathers to analyze the situation, but they are interrupted by
a man covered in sores creating commotion in the village, saying that the creature attacked him and
his men. Before they can ask him anything, the man dies in front of everyone. Yun-kyum and Sung Han
return to the shamans’ scene, finding one of them on top of some trees, wondering how it got there.
Meanwhile, Myung and Hur investigate the man with sores’ body. The girl doesn’t seem so
disturbed by it, and Hur takes this as a sign of courage. She explains that being so bored at home,
she would read any book she could get her hands on, acquiring knowledge in medicine to be a female
doctor in the future. Out of nowhere, Myung tells Hur that her new mission will be to find a rat,
leaving the young man bewildered. Yun-kyum reveals to the king that the deaths were indeed done by a
human. The only one who disagreed to this was the man with the sores, who claimed to see Monstrum,
raising suspicions of his veracity and his relation to the plague. He also shows that they
used a sample of the man’s blood and transfused it into a rat, which died in only half a day,
showing the plague’s lethality. While the king thinks it’s all a plot to overthrow him,
Yun-kyum assures him that Monstrum is real in the sense of danger to the population,
so they have to find the truth. The king’s council asks him to use the tiger hunters to
investigate Mount Inwangsan. He agrees but only asks for 100 out of the 700 available soldiers.
Before anyone can object, the new search commander, Yun-kyum, makes his appearance
in his old soldier’s outfit, leaving more than one surprised. The soldiers begin to collect
resources in the city, but this is difficult due to the villagers’ poverty, who without their food,
won’t be able to survive that season. If they cannot provide enough, they would be used as
temporary warriors, something that weighs heavily as they aren’t experienced fighters.
Sung Han finds a couple of boys that claim to be the only men in their family, so he drives them
out of the place, but not without giving some food. On his side, Yun-kyum expels an old man,
but he refuses in order to do his civic duty, not without much hesitation. Upon closer inspection,
Sung Han notices an intruder in the ranks of the soldiers. Myung had disguised herself as a man to
go unnoticed, as she considers herself good enough to fight. Yun-kyum tells her that this mission is
not a game, but before adding anything else, the arrival of the Prime Minister is announced along
with Jin Yong, the leader of the tiger hunters. They all start inspecting with no results, and
when they discuss whether to continue at night, Yun-kyum decides to do so, as it is known that
the creature tends to appear more at those hours. The hunters and villagers split up to cover more
territory, and after seeing the old man separated from the group, Sung Han decides to go in search
of him. He ends up in the middle of a rock wall but is soon followed by members of his group,
including Yun-kyum, Hur, and Myung, the latter taking an interest in the site. Having been so
small, she doesn’t remember that it’s the same place where her mother died years ago and where
Yun-kyum rescued her. She only asks her father if she has been there before, but he doesn’t answer.
Hur finds a footprint of a giant animal, which leads to a cliff in front of the rock wall.
Elsewhere, the villagers lead the march to the tiger hunters, but suddenly the
hunters treacherously attack the villagers, taking them off. They plan to set a trap for
Yun-kyum’s group by setting off a false flare as he climbs down the cliff to look for the beast.
As they are about to pull him up again, the enemy attack begins, making it challenging to
help Yun-kyum. Hur and Myung manage to defend themselves, and during the battle, Sung Han
manages to pull Yun-kyum up so that Yun-kyum can start fighting. The four mains regroup,
leaving only them standing. Yun-kyum orders Myung and Hur to alert the king while he and
Sung Han take care of the rest of the hunters. Both men fight bravely, but when the commander
is about to be shot by an arrow, Sung Han steps in to save him, taking the attack in the arm.
Both are captured and surrounded by the enemy. Jin Yong appears and confesses that the monster
doesn’t exist, and the rumor was created to eradicate the king’s followers and army.
The hunters also manage to subdue Myung and Hur, tying them together. Suddenly, Myung has
a regression of the past, remembering how it was Jin Yong the one who killed her mother years ago.
He also remembers her from when she was a child, but before speaking further, he throws her and
Hur off the cliff, causing Yun-kyum’s anger, and swears to Jin Yong that he will take revenge for
that. The young couple managed to survive by hanging on to a rock during their fall,
but with no chance of getting out of it safely. Jin Yong sends another flare to alert the king of
the beast’s existence, which will be an alibi to justify the death of all the villagers.
A wounded lion-like beast emerges from the cliff, and although several arrows are shot at it
simultaneously, the beast doesn’t even flinch. It goes full force onto the battlefield, annihilating
everyone. Sung Han and Yun-kyum manage to escape and decide to go to rescue the others.
They find them at the bottom of the cliff, this being the beast’s nest covered with his victims.
They decide to go out and alert the king, but just then they meet the old man, who tells them
that they should all smear some of the goo from the nest to go unnoticed, covering their scents.
The old man tells them that the beast has terrible eyesight but excellent hearing, so they all keep
quiet. They avoid the beast, at least for a while, until Yun-kyum tells them all to run through the
cave passages. They fall into a river, with the beast no longer chasing them. Back at the base,
Jin Yong tells the Prime Minister that Monstrum is real, even though the Minister tells him that
it was his intention from the beginning. He orders all the tiger hunters to go to the city and set it
ablaze to overthrow the king. A couple of wounded villagers manage to reach the city, alerting the
other villagers about Monstrum’s attack. Jin Yong appears to start the fire, which gradually spreads
to the point where it can no longer be contained. Guided by the old man, the group ends up in the
drains of the royal palace, and from there, they arrive at the Jojun Hall, a giant prison.
He explains that the former king Yeonsan loved exotic beasts, so he kept them locked up there. He
used to work for the king as an animal feeder, and reveals to have known Monstrum since it was just a
cub, initially calling it ‘Sparkles’ because of its sparkly eyes. When Yeonsan was overthrown,
they got rid of all the beasts except Sparkles, who was rescued by the old man, freeing him
and fleeing down the drain, where he fed on rotting food and turned into a giant monster.
Everyone is surprised when the Monstrum appears in the living room, but while everyone flees, the
old man stays behind distracting the beast, making it remember its days as Sparkles. He leads it to
a cage with the help of an old rattle, but ends up locking himself too so that the others can escape,
sacrificing himself to the animal’s wrath. The Prime Minister orders the tiger hunters
to burn everything to eliminate the infected bodies, and everyone considers that the king
has no more allies inside the city. The main group manages to get out to the royal palace, knowing
that the Monstrum is below them, so they have to alert the king. Yun-kyum does this in private,
but they are surprised by an ambush by the Prime Minister, who wounds the king with an arrow.
Yun-kyum is surrounded, his allies captured, the royal guard is wiped out, and the beast is
on its way to top it off. The Monstrum breaks into the royal hall and eliminates the hunters,
including Jin Yong, giving Yun-kyum and the king a chance to escape. Sung Han and Myung manage to
escape by pretending to be sick with the plague, so they search for Yun-kyum. The beast manages
to get out of the royal temple and begins to pick off all the soldiers, despite their best efforts
to counteract it. Still, they subdue it with ropes and hooks while Yun-kyum meets the royal
guard and asks them to escort the king to a safe area. The Minister decides not to kill the beast
but to use it to terrorize the people and thus seize the throne. Yun-kyum meets with the others,
and they devise a plan to eradicate the beast, consisting of explosives and the Jojun hall.
The beast breaks loose and continues its attack, with the Prime Minister fleeing as his men fall.
The townspeople try to enter the palace territory, but everyone inside tells them to do otherwise,
as Monstrum attacks everyone. They try to close the gates to lock the beast in,
but just then, the Prime Minister appears, angry at such deliberate action. However, the people
realize the Minister’s betrayal, so they turn against him and lock him inside the palace.
Monstrum appears and starts chasing the prime minister, who runs to the top of the wall to
show all the villagers below them. He tries to negotiate with the beast to leave him alone, but
it lashes out and devours him. Yun-kyum appears to shoot an arrow at it, which hits it squarely,
knocking the monster to the ground. Remembering the old man’s words, Yun-kyum guides the beast
to the main hall using its hearing. Meanwhile, Sung Han and Hur fill the place with explosives
for the encounter. This gets complicated when Jin Yong reappears to initiate a fight.
Yun-kyum and Myung meet at the entrance of the royal temple, barely dodging the beast’s attacks.
He tells her to go inside while he draws Monstrum’s attention to fall into the trap.
Sung Han and Hur are losing their fight, and when Jin Yong goes to finish Sung Han, Hur steps in,
receiving the sword cut, but tackling the enemy. Myung appears in the room and aims at Jin Yong,
but he uses Hur as a shield. The girl tells Hur to remember the day they met and takes the hint,
with the boy dodging the arrow just in time. Jin Yong falls, and Hur confesses to Myung
that he will never forget the moment they met. She shouts to her father that everything is ready for
the explosion, so the commander runs one last time to jump into the Jojun hall holding on to a rope,
making the monster fall. Han Sung lights the fire and everyone flees. Just before making it,
the beast manages to extinguish the fuse’s flame. Yun-kyum pushes Sung Han and gets himself locked
inside the room. His brother just says goodbye and orders Sung Han to take care of his daughter.
The beast hits Yun-kyum in the fight, but he reaches the explosives, knowing how his fate will
end. Myung notices her father’s absence, and when she asks to return for him, the explosion begins.
The rain extinguishes the flames, and from the rubble, the beast closes its eyes for the last
time. Everyone mourns the loss of Yun-kyum, and the villagers begin to reenter the village with
the king, seeing the soldier’s noble sacrifice. However, Myung keeps searching through the rubble
until finding her wounded father. They all help him free from his concrete prison and celebrate
that they could finally get rid of the beast. The epilogue explains that the king moved his
residence to another palace, not returning to the previous one until three years later. Time passes,
and Myung and Hur, now a couple, are separated from royal soldiers Yun-kyum and Sung Han.
The latter asks Yun-kyum how he managed to survive the temple explosion, and a flashback shows how he
was helped by a pulley system that lifted him into the air just before everything exploded.