In the kingdom of Aurea, the king and his knights ride through the hills as they chase a dragon. They follow the beast into a cave, where the group sees something that leaves them in shock. At that moment the dragon flies in and breathes fire to kill most of the knights in one go, then she uses her tail to capture the remaining knights one by one and kill them too.
Lastly she approaches the king, who falls to his knees to wait for the end. Many centuries later in a land in the north, Floria and Elodie are chopping whatever wood they can find. They’re the daughters of the local lord and are trying their best to help their people, who are suffering from hunger and cold because the winter is ravaging the land.
More and more people are leaving every day, which means they may not have a town at all soon. Suddenly the sisters see a carriage come by, which is strange because they never get visitors. They rush back to their home to find their father Lord Bayford and stepmother Lady Bayford talking to a nun, who says Elodie will do before leaving.
Lord Bayford then tells Elodie that the royal family of Aurea wants her to marry their prince. At first Elodie refuses, but Bayford changes her mind when he points out that their town will receive enough money to save their lives for many years to come. Sometime later the family sails to Aurea.
When they get close to their destination, the ship enters a very thick mist and the group gets scared of some creepy figures in the dark, but they turn out to be two giant dragon statues. Once they land, the family is amazed by how prosperous the land is and the magnificence of the castle. The servants take them to their rooms and inform them they’ll meet the royal family the next day.
Elodie goes to the balcony to admire the view and notices on another balcony a girl her age, who is soon taken away by a maid. That night, Elodie can’t sleep so she keeps herself busy drawing. Then she goes to the balcony to get some fresh air and notices there are no lights in the other girl’s room.
She’s also surprised to see torches in the distance, meaning someone is going up the mountain in the middle of the night. The next day, Elodie’s family meets the royals. Queen Isabelle sends Elodie to spend some time with Prince Henry before the wedding.
After the initial awkwardness, Elodie and Henry hit it off and have a very pleasant conversation. After Henry compliments Elodie’s artistic skills in her letter, Elodie asks about the torches she saw last night. Henry explains it’s one of three ancient ceremonies that honor their ancestors, that’s why the wedding is on the same day.
Meanwhile in the castle, Lord Bayford finishes discussing the deal with Isabelle and looks very upset. Lady Bayford tries checking on him, but he snaps and promises nothing is wrong. Afterward Lady Bayford tries to chat with Isabelle, who doesn’t take it well.
When Lady Bayford explains she thinks the families should bond and get along, Isabelle says this is only a transaction born out of necessity: she needs a princess and Bayford’s land needs money. She also reminds Bayford that she’s the daughter of a rope maker so she shouldn’t forget her station. Lady Bayford notices that Isabelle says Elodie’s name wrong and tries to correct her, but Isabelle doesn’t care and leaves.
In the evening, a worried Lady Bayford goes to tell Elodie that there’s something suspicious going on, so she thinks the wedding should be canceled. But before they can discuss things further, Lord Bayford shows up and sends everyone to bed. The next morning, the maids help Elodie get ready for the ceremony.
She’s given a beautiful dress that includes a blunt dagger and a diffuser with perfume. The wedding goes well: Henry and Elodie exchange rings, share a kiss, and Elodie is given her own crown. Afterward Elodie says goodbye to her family and gets on a carriage with Henry, who explains they’ll join the ancient ceremony on the mountain.
They go up a rocky set of stairs and Elodie is shocked to find a bunch of people wearing creepy golden masks, not to mention Isabelle dressed like a nun. The queen gives Elodie a coin saying she’ll be helping protect the kingdom, then they go inside as Isabelle shares a story. When their ancestors arrived on this island they discovered a dragon lived there as the last of its kind.
The dragon attacked the village as soon as it saw the humans settle down, so the king gathered his soldiers and tried to fight the beast. However the dragon killed everyone except the king, who had to give up his three daughters so the island could be shared. As she listens to the story, Elodie notices how everyone stares creepily at her and the fact they’re crossing a dangerous bridge above a chasm.
Then Isabelle explains the king’s sacrifice is what they celebrate every generation before cutting Henry’s and Elodie’s hands. She pushes them together to mix their blood and covers them with a cloth, announcing that Elodie is now officially of royal blood. Next Elodie throws the coin into the chasm, thinking it’s the end of the ritual.
Henry picks her up to carry her through the bridge, only to suddenly apologize and throw her into the chasm too. Screaming as she falls, Elodie hits a bunch of brunches until she lands on a mud puddle and passes out. She wakes up moments later with various bruises on her body, but she still stands up and starts yelling for help.
There’s nobody left on the bridge, however her voice does wake up the dragon hidden deep in the cave. Then Elodie cries as her desperation turns into fury and she tears the fancy jewelry off before trying to climb out. Unfortunately she falls again and gets another injury.
At that moment she sees other jewelry that doesn’t belong to her and realizes the royals have been sacrificing girls to the dragon for years. Suddenly she hears a noise and turns around to notice a light inside a cave, so she goes inside to find a bird on fire. Elodie immediately uses some dirt to put the fire out, but it’s too late: the bird is already dead.
Suddenly a roar echoes in the cave and hundreds of birds on fire come out, flying above Elodie for a few minutes before they drop dead around her. With the little bodies serving as lights, Elodie tries to look for an exit only to freeze when she sees the dragon coming. Elodie runs to hide behind a stalagmite as the dragon starts talking, saying that every generation of royalty must pay and that she can smell it in Elodie’s blood.
Looking at her scar, Elodie realizes the ceremony is done to trick the dragon. The dragon tells Elodie to run and she does so as she screams, managing to enter a tunnel as the dragon breathes fire behind her. The flames enter the tunnel too, but Elodie manages to find a safe spot just in time.
There’s a burnt body there, and she realizes it’s the girl she saw on the balcony. When she hears the dragon coming after her again, Elodie tries to escape through a small hole, only for her dress to get stuck in it. The flames are getting closer, so after lots of struggle Elodie manages to remove the crinoline and run away, hiding in a smaller cavern.
Unfortunately some fire still reaches her leg and burns it, causing her to scream in pain. The dragon hears her but can’t reach her because there’s not enough room for her giant body. Once the dragon is gone, Elodie remembers the blunt dagger and hones it against a rock to then cut off part of her dress.
The pain in her leg is unbearable but she pushes through and bandages the wound with the torn fabric. Next she opens the diffuser and catches a bit of fire in it to use it as a lantern. Now she can explore the cavern and eventually finds a very narrow passage.
She begins crawling through it and her dress gets stuck in it again, so she pulls until she’s freed and slides down in the process. Elodie tries her hardest to hold on but her hands are wounded and tired, so she falls again and the fire in the diffuser goes out. Refusing to give out, Elodie stands up and holds onto the wall to walk, determined to reach a light she sees in the distance.
After some very painful walking she leaves the tunnel, only to almost fall off the edge of a cliff. She can still see the light on the other side, so after taking off her wedding ring and throwing it into the abyss, Elodie steps back and runs to make a big jump. The other side of the cliff is slippery and she can’t hold on, but luckily as she slides down her belt gets stuck in a rock.
It soon breaks, however it gives her enough time to take out the dagger and stick it into a crack to hold on. Then she uses the dagger to climb up the rest of the way and discovers the light comes from cute glowing slugs. After checking they don’t bite, Elodie tears off her puffy sleeve and puts a bunch of slugs inside to make a new lantern.
Then she walks a little further and finds a small lake, but when she tries to drink, she immediately spits the water out. It turns out the cave plants give it a disgusting taste. However there is also dripping ice on the ceiling, so Elodie stands under it to catch any drops she can.
Suddenly the ice starts melting faster and the light coming through it looks a bit red, so Elodie opens her eyes to discover the dragon breaking through the ice with her fire. Elodie runs away as the dragon breaks through the ice and lands in the tavern to chase after her, once again not going far because Elodie enters another tunnel too small for giant beasts. In this small tavern, Elodie finds a message on the wall that says “safe here, she cannot reach” and is signed by the letter V.
As she looks around, Elodie finds the dresses of many girls and on another wall, their names added through the years, including the V that belonged to a girl named Victoria. Elodie takes a moment to grieve for them before removing the bandage off her leg, which isn’t looking any better. Afterward Elodie tries to sleep while hugging the lantern.
Suddenly she starts seeing the ghosts of the previous sacrifices, suffering in pain, fear, and betrayal. Victoria is adding her name to the wall only to suddenly turn around and tell Elodie that it’s all a lie. At that moment Elodie wakes up and finds slugs all over her leg wound.
She freaks out and starts removing them, only to discover their slime has healing properties and now her leg is fine. She puts a slug on her other major wound and apologizes for underestimating it. Feeling better, Elodie takes a better look around and finds a map on the wall, which means at least one girl managed to get out.
The map teaches her she’ll find three forks in the tunnel and she must take the middle one, where she must follow the crystals and the music to the outside. Afterward Elodie tears off all the inconvenient parts of her dress, adds her name on the wall, and takes the middle fork while ignoring the echoes of the dragon's growls. Eventually she hears a little tune and walks a little further to find a wall covered with crystals, which are the cause of the noise and also glow under the rays of sunlight because they’re right under the exit.
Nearby there’s a crown with a V on it, which means it was Victoria who got out. Since the crystals are pointy, Elodie tears off more of her dress to cover her hands and feet, then with the help of the crown she starts climbing up. Unaware that the dragon is watching, Elodie makes an effort to go up and a crystal breaks under her hands causing her to fall, but she uses the crown to hold on at the last second.
Then Elodie hears the dragon nearby, so she hurries up and finally reaches the exit, only to discover she’s on the edge of a hole very high in the side of the mountain. Devastated, Elodie falls to her knees and cries. Suddenly the sound of horses neighing help Elodie notices a group of riders approaching the mountain.
She tries yelling for help, but at that moment the dragon flies toward her and covers the hole. Elodie steps back just in time and notices another message on the wall saying “not safe” signed by V. Nearby there’s a body, meaning Victoria died there and never got out.
The dragon gets her fire ready, only to stop when she hears a male voice calling out Elodie’s name. While the dragon flies off to investigate, it’s revealed that Lord Bayford has come with two knights and a guide to look for Elodie. They descend into the cave with some rope and start searching the area, so Elodie uses the sound of their voices to find her way back.
When she takes a new tunnel, she’s shocked to find a nest with three dead baby dragons and remembers Victoria’s words, which makes her realize Isabelle had been lying: the dragon hadn’t been the last of her kind and she hadn’t struck first. A flashback reveals that the king had entered the tavern precisely to kill all the babies, so the dragon’s attack had been an act of revenge. She killed all the knights as seen in the beginning, but when it was the king’s turn, the dragon told him death would be too easy and he should suffer the same way she did.
Since then the royal family has been feeding the dragon three women with their blood mixed to make them pass as royal daughters. Afterward Elodie finds her father and his men, but she doesn’t show herself yet for safety. The dragon shows up and grabs a knight, taking him for a flight before dropping him from a great height.
The knight bounces on the rocks and crashes to the ground, instantly dying. While the guide rides to hide, the dragon grabs the second knight and crushes him under her paw before interrogating Bayford. He explains he’s here to save his daughter and tries to attack with his sword, but the dragon grabs him with her tail and disarms him before dropping him.
While Elodie sneaks around waiting for the right time, the dragon tells Bayford to call for his daughter. Bayford decides to offer a heartful speech about how sorry he is and admitting he made a horrible mistake, then he yells to order Elodie not to come out, causing the dragon to push him down and stab him right in the heart with her claw. Elodie can’t help making a noise as she cries and the dragon starts searching for her, but luckily at that moment the guard also makes a noise and the dragon goes after him, thinking it’s Elodie.
With the dragon gone, Elodie says goodbye to her father and kisses his forehead before he dies. Then she uses the rope left by him to start climbing out of the tavern. Meanwhile the dragon finds the guide and crushes him to death before coming after Elodie, who manages to climb fast enough to avoid the flames that now cover the entrance.
Elodie immediately gets on her father’s horse but only rides for a few moments before jumping off to hide under some rocks. Since the horse keeps going, the dragon is tricked and continues to follow it to burn it to death. Then the dragon breathes fire on the clouds, causing a horrible storm as a sign the deal has been broken.
In the castle, Isabelle notices this and immediately sends her knights to kidnap Floria. Lady Bayford tries to stop them, only to get stabbed in the process. Moments later when the fire is over, Elodie comes out of hiding and bumps into Lady Bayford, who came all the way here on a horse while still bleeding.
When Elodie hears what happened, she promises to save her sister and takes the horse back to the mountain. Meanwhile Floria is taken to the bridge, however Henry refuses to do the ritual with a child. Isabelle ignores him and uses her own blood instead, cutting Floria’s hand to do the mixing before throwing her into the chasm.
By the time Elodie arrives at the bridge, everyone is gone and an unconscious Floria has been caught by the dragon, who doesn’t kill her yet because she wants to bait Elodie into coming for her sister. A desperate Elodie descends back into the tavern with the rope and uses her previous experience to follow the same path much faster. After double-checking the map to find the dragon’s lair, Elodie arms herself with her father’s sword and prepares a trap for the dragon.
This trap consists of some armor and rope, which is burning under a small flame. Elodie walks away and when the rope finishes burning, the armor falls, causing the dragon to fly out of her lair to look for the source of the noise. This allows Elodie to sneak into the lair and wake up Floria, who walks slowly because she’s got a wounded leg.
At that moment the dragon discovers the trap and comes back to the lair, so Floria hides while Elodie waits with her sword out. She tries to explain that they've all been tricked by the queen, but the dragon doesn’t believe her and tries to breathe fire. Elodie stabs her mouth with the sword to redirect the attack but still falls off the edge of the lair, receiving some burns on her arms and shoulder in the process.
While the dragon removes the sword, Elodie lands in a lake and swims out just in time for Floria to throw another sword at her. Suddenly the dragon jumps on her and holds her while stabbing her with her claw. Before she can kill her, Elodie takes out the dagger and stabs the dragon in the eye, causing her to throw Elodie away.
Next Elodie grabs the sword again and runs toward the dragon, who has trouble seeing with one eye. She manages to stab the beast in the chest, however it doesn’t cause much damage and the dragon grabs her again. Elodie then stabs the dragon’s paw, making her drop her again.
As the dragon comes closer, Elodie notices the dust bouncing against a curved stalagmite, giving her an idea. Elodie stands in front of the stalagmite and tells the dragon to burn her down, so the dragon breathes her fire. However Elodie moves at the last second and the fire bounces on the stalagmite, causing it to land on the dragon and finally bring her down.
Afterward Elodie shows the dragon the scar on her hand to prove that the beast has been killing innocent girls, not royals. The dragon asks her to end it, but Elodie drops the sword and refuses to continue this circle of hatred. Then she grabs a bunch of slugs and puts them on the dragon’s body so she can fully recover.
Meanwhile in the castle, another wedding is being celebrated so they can trick the third girl of this generation. Suddenly a coin comes rolling down the altar and Elodie walks in, telling the bride to take her family and run away. Then Elodie tells the locals they can also run away if they want, but only the servants take her seriously and leave.
At that moment the dragon shows up and breathes fire on everyone present, killing Isabelle and every person in the royal line to finally end it. As everyone panics, Henry closes his eyes and accepts the punishment because he knows he deserves it. Soon the castle crumbles down but Elodie walks away with her head held high.
A few days later, Elodie, Floria, and a recovered Lady Bayford get ready to leave in a ship full of gold and supplies for their people. They agree to run their land together, and Elodie even starts calling Bayford “mother” instead of “stepmother”. During the trip home, the dragon flies nearby and shares a nod of mutual respect with Elodie.