Pinmei is a shy girl who lives with her grandmother, Amah, in a rough stone hut on the side of the mountain. Yishan, a boy barely older than Pinmei, lives farther up the mountain. While villagers regularly climb the mountain to request that Amah embroider their silk fabrics for special occasions, they stay for Amah’s stories. Lately however, a bitter, unrelenting winter has kept many of villagers away.
There have also been rumors that the new emperor captures and enslaves men from local villages and forces them to build a vast wall that surrounds the entire kingdom. It is tradition for all new emperors to pay tribute to the mountain and get the blessing from the Mountain Spirit on top, but while the fierce and brutal Tiger Emperor tried to climb the mountain, the Mountain Spirit determined that the emperor was not worthy and forced him back down.
One night, Amah hears soldiers approaching her hut and hides Pinmei in a barrel. The Tiger Emperor himself, disguised as a soldier, orders his men to take Amah and burn the hut. Yishan comes down from his hut, confronts the emperor and demands Amah be released, but the emperor easily picks him up and throws him to the ground. The emperor tells him Amah will be returned if Yishan can bring him the Luminous Stone that Lights the Night. The soldiers leave, and Yishan rescues Pinmei before her home burns to the ground. The children plan to find the Luminous Stone so they can get Amah back.
The next day, Yishan and Pinmei go down the mountain to the nearest village, where they find sorrow and destruction. Soldiers have taken all the able-bodied men from the village to work on the Great Wall. As before, the emperor tells the villagers that the men will be returned if someone brings him the Luminous Stone that Lights the Night. Remembering one of Amah’s stories about a king in the City of Bright Moonlight, who had a dragon’ pearl, Pinmei and Yishan hope the pearl is the Luminous Stone and decide to go there so they can save Amah.
On the way to the City of Bright Moonlight, the children meet a beautiful woman also on her way to the city. Lady Meng’s husband, Wan, went to see the Tiger Emperor on behalf of the king of the City of Bright Moonlight, and he has not returned. She fears her husband is dead and cries into Yishan’s handkerchief.
Lady Meng tells the children that she feared for her husband’s life so she sewed him a dragon shirt to protect him, leaving her needle in the shirt. In the City of Bright Moonlight, King KaeJae tells Lady Meng that when the emperor saw Wan, he grabbed the man by the shirt, ripped it and ordered Wan taken away to work on the vast wall. Shortly after, the emperor became invincible. Before they can ask more questions, the king urges them to hide, as the Tiger Emperor has arrived, and warns them not to go into the palace gardens.
That night, the children leave their hiding spot to search the soldiers’ tents for Amah and unintentionally go into the garden where they overhear the Tiger Emperor and King KaeJae. The king offers the emperor the dragon’s pearl, but it’s not the stone he seeks. The king is confused, and the emperor implies that if the king went to the bottom of the sea, he would understand.
The Tiger Emperor demands the king consult the Paper of Answers, a magical object that gives the king one answer during the full moon. Previously, the paper had responded that the secret to immortality was stories, leading the emperor to take Amah, the Storyteller. For his one question, the emperor asks the paper if he will achieve immortality, and the paper responds yes.
The emperor and his men leave, and the king gives the Paper of Answers to the children, who decide to travel to the Sea King’s crystal palace at the bottom of the sea to find the Luminous Stone. While Lady Meng promises to show them the way, she cannot accompany them, as she must go to the wall to find her husband.
Pinmei and Yishan ride Lady Meng’s magical horse to the sea where it takes them beneath the freezing water to a beautiful underwater palace owned by the fierce Sea King. When they arrive in the Sea King’s court, he thinks they will help with the Black Tortoise of Winter’s dilemma. The tortoise has sent a message asking for help, as he is trapped and unable to return to the Starry River (the heavens), and so the brutal winter being experienced by the mortal world cannot end.
Pinmei explains the true reason they are there: to find the Luminous Stone that Lights the Night. When the goddess with the fish tail, Nuwa, sacrificed herself to save the sky, the earth and the seas, she left behind three things: a strand of her hair, also called the Iron Rod; a drop of blood, known as the Red Stone; and a single tear, named the Luminous Stone that Lights the Night. The Sea King explains that all three of Nuwa’s items are in his dominion.
The king takes the children to a bridge that flows over an endless body of water, the Starry River, which is actually the heavens itself. He explains to the children that the Sea Bottom is where both worlds connect; the bottom of the Starry River is the sky for mortals. He points down in the river to Nuwa’s tear, the Luminous Stone that the Lights the Night, the moon.
Amazingly, Yishan is able to use a string from his shirt to lasso the moon and pull it toward the bridge, but the Sea King and Pinmei convince him that the moon cannot be for the corrupt emperor alone. It has to be for everyone, so Yishan puts the moon back.
Without the stone, Pinmei and Yishan need another way to save Amah. So they consult the Paper of Answers. Instead of asking about Amah, Yishan wants to know how the emperor captured the Black Tortoise of Winter. The answer they get is that he did it with the Iron Rod.
Pinmei is upset about the wasted question, but Yishan asks for her trust. Pinmei has become increasingly alarmed at Yishan’s boldness, knowledge and new abilities. Yishan has realized that the emperor’s enormous power and the endless winter must come from the Black Tortoise.
The Sea King orders his treasury searched for the Iron Rod, reminding his guards that it can shrink to the size of a needle and has been used by his queen and her handmaids for sewing. When it’s confirmed that the Iron Rod is indeed missing, Pinmei and Yishan ask the Sea King to take them back to the mortal realm. He transforms into a majestic dragon and flies them through heavens and back into the mortal realm.
Pinmei and Yishan arrive in the capital city to see a funeral procession for Lady Meng’s husband. She found his body at the wall, and in her grief, destroyed a portion of it. The emperor was angry and wanted to punish her, but when he saw how beautiful she was, he wanted to marry her instead. Lady Meng agreed but insisted on burying her husband at sea first. As soon as his coffin is put into the sea, she jumps in herself. While others believe she jumped into the icy waters to her death, the children look and see Lady Meng swimming, her legs transformed into a fish tail.
While the children try to figure out what to do next, Pinmei despairs over how to rescue Amah and begins to cry. Yishan comforts her and offers his handkerchief, just like he did for Lady Meng when they first met. Pinmei notices a small, shining luminous stone in the handkerchief.
Lady Meng is the Sea King’s daughter, a goddess with a fish tail just like Nuwa, and her tear is a Luminous Stone that Lights the Night. Lady Meng fell in love with and married a mortal. When her husband went to see the emperor, she feared for his life, so she sewed a shirt for his protection and left her needle in the shirt, the Iron Rod she got from her father’s treasury. The emperor took the needle and trapped the Black Tortoise.
Armed with the stone, Pinmei and Yishan beg the guards at the palace gates to let them see the emperor. The emperor sees the Luminous Stone and agrees to trade Amah at nightfall, when the stone can be tested. They meet at the Black Tortoise gate, and soldiers bring Amah, looking thin, tired and gagged.
The emperor demands that Pinmei tell a story, and she does, telling how the emperor tried to climb their sacred mountain but was stopped by the Mountain Spirit who knew that he had bound the Black Tortoise with the Iron Rod. Determined to keep his invincibility and seek immortality, the emperor decides to build a vast wall to hold the Black Tortoise, in case the Iron Rod slips.
The emperor is furious and confronts Pinmei, who notices an embroidered image of a tortoise speared through by an iron needle on his uniform. Before Pinmei can grab the needle, Yishan holds up the glowing stone; the emperor is delighted.
Amah gets off the gag in time to shout that it’s a trap. The emperor doesn’t want the stone. Instead, he wants the only immortal who can carry the stone and whose death will grant him immortality — the one known as Ginseng Boy, Man of the Moon or the Mountain Spirit. As the emperor orders guards to capture Yishan, Pinmei quickly grabs the needle and yanks the Iron Rod off the Black Tortoise of Winter.
As the embroidered tortoise unravels, an enormous burst of wind and winter whips though the air. Earthquakes shake the ground, and everything around Pinmei is destroyed. A portion of the palace gate falls on the emperor, killing him. Pinmei sees Amah lying hurt, so Yishan takes the needle and pricks his finger, bleeding a drop of clear, golden liquid into a bowl for Amah to drink.
Amah recovers, and Pinmei is shocked to see that Yishan has transformed into an old man. Yishan, an immortal who transforms into a young boy every 99 years, has given up his youth to save Amah. Yishan leaves them, and Pinmei and Amah leave the palace to go back home.