Fourteen-year-old Jin Ling has disguised herself as a boy and has lived by three rules in order to survive in Hak Nam, the Walled City: Don’t trust anyone; always carry your knife; run fast. She entered the city to find her sister, Mei Yee, who was sold into one of the brothels by their abusive father.
When Jin steals a pair of boots from Kuen — the leader of one of the many gangs that roam the streets — she flees down the narrow alleys in order to escape. Her quickness attracts the notice of Dai — an older teenager haunted by his past and desperate for a chance to return to his life, even though he does not feel he deserves it. He needs a fast runner in order to infiltrate the Brotherhood, the kingpins of the city, led by Longwai.
Seventeen-year-old Mei Yee lives a hopeless life in Longwai’s most prestigious brothel. Young and beautiful, she has become the favorite of an ambassador from the city outside the walls. He pays Longwai handsomely to keep Mei Yee exclusively for himself. Mei Yee and her friends must watch in horror as Sing, one of their fellow prostitutes, is brought back after she tried to escape. Instead of beating her, Longwai injects her with heroin, thus ensuring Sing will become an addict. The once vibrant girl is reduced to a shuddering shell of her former self. The message to the other girls is clear. They belong to Longwai, and there is no escape.
Jin agrees to help Dai only as a way to enter the Brotherhood’s brothel. It is one of the few she has not been able to enter in her search for her sister. Jin must take an envelope to a man in City Beyond, the city outside. He will give her money in exchange. If she does not return, Longwai will kill Dai.
Jin proves her tenacity when the customer does not give her the money for Longwai’s package. She stabs the man’s arm, retrieves the package and returns to the brothel. Longwai is impressed. He offers the boy a regular job. He wants Jin to deliver drugs for him while Dai is kept in the brothel as collateral. The two agree to Longwai’s terms because they both secretly hope to find something within the brothel: Dai wants information; Jin wants to find her sister.
Dai returns to the brothel alone and finds a window to one of the prostitute’s rooms. It belongs to Mei Yee. She is drawn to his eyes and youth. They begin a tentative friendship through the glass. Dai needs information about Longwai and the Brotherhood. If he provides it to the police within two weeks, they will allow him to leave the Walled City.
If Dai does not provide the information, he will be forced to pay for a crime he committed two years earlier. He had gone to buy drugs with a friend. The buy went bad and his friend and the drug dealer were killed. Caught in the crossfire was Dai’s younger brother, who had followed to stop Dai from doing drugs. Dai considers his time in the Walled City his punishment for the part he played in his brother’s death. Dai promises to bring Mei Yee a seashell if she will help him get the information he needs about the Brotherhood.
Jin does several successful runs for Longwai, before Kuen, the boy whose shoes she stole, finds her. Before Kuen and his gang can beat her, Dai arrives. He threatens to shoot the boys if they hurt Jin. The gang leaves, but both Dai and Jin know they will return.
Dai offers to let Jin stay at his apartment. Jin is surprised to learn that Dai is not homeless. She worries what other secrets Dai may be keeping, but sleeping indoors is a luxury she cannot pass up. When Dai leaves in the middle of the night, Jin follows. She eavesdrops as he meets with a man who passes him a package. The man talks about a deal Dai has made with the Security Branch, but warns him not to get into trouble.
The ambassador asks Mei Yee if she would like to live with him in an apartment. Before she met Dai, she would have jumped at the chance to leave, but now she dreams of a life with Dai. When she tells the ambassador she must think about his offer, he becomes enraged, believing she must be sleeping with another man. She convinces him she only hesitates because it would be hard to leave her friends.
Kuen and his gang find Jin again. This time Jin stabs and kills the older boy, but not before she is also stabbed. One of the younger gang members finds Dai to let him know. Jin is barely alive when he finds her. He must get her to a doctor outside of the Walled City if she is going to survive. There is only one place he can safely take her, and that is to his old home. His parents, although fearful the police may catch him, allow the two inside. It is only after the doctor sews up her wound that Dai finally learns Jin is a girl. Dai must return to the Walled City in order to get Longwai’s ledger for the Security Branch, but Jin is too weak and must recuperate with his parents. Jin begs him to try and find her sister, Mei Yee. Neither know that Mei Yee is the girl he has been speaking to through the window.
The ambassador asks Mei Yee if she has made up her mind about living in an apartment. When she refuses him, he brutally rapes her. She covers her bruises and continues to spy on Longwai for Dai. She discovers where Longwai hides his ledger. When Dai returns, he tells her his partner has been injured, and it affects his plan to get the ledger. Without the ledger, he cannot be free, and he cannot help her escape. Mei Yee decides she would rather risk death than stay a prisoner in the brothel. She agrees to steal the ledger for Dai. They tell each other their names, and Dai is shocked to learn she is Jin’s sister.
Jin returns to the city so she can help Dai. When she and Dai return to the brothel, they overhear Longwai discussing Mei Yee. One of the other girls has told him of her relationship with a boy outside the brothel and their plan to escape. Longwai does not know who is telling the truth, but he calls for Mei Yee to be brought to him. Dai believes the ambassador may help save her life. He gives Jin his gun and tells her to bring the ambassador to the brothel.
Meanwhile, Dai is brought before Longwai because he missed several drug runs. Dai explains that his runner was injured. He convinces Longwai that it will be in his best interest to keep him alive. Longwai likes his tenacity and agrees to put Dai to the test. If the boy passes, he can officially join the Brotherhood. Unfortunately, the test is to help question Mei Yee. When she will not divulge to whom she has been speaking, Longwai gives Dai a vial of heroin with which to inject her.
As he approaches Mei Yee, Sing lunges forward and steals the syringe. In the chaos, the ambassador arrives. He insists Longwai sell Mei Yee to him. He also realizes that Dai is the son and heir of Sun Industries and announces it to Longwai. Longwai shoots Dai in the shoulder as the ambassador pulls Mei Yee from the room. They hear a second gunshot and believe Dai has been killed.
Jin convinces Kuen’s old gang to help her stop the ambassador. The boys set upon the man like wild dogs. Jin and Mei Yee try to escape. They see one of Longwai’s men dragging out a body from the brothel. Both are relieved to learn it is not Dai’s body, but Sing’s. The two come up with a plan to free Dai. They exchange clothes so that Jin can enter the brothel unnoticed.
Several of Mei Yee’s friends help Dai to escape the room in which he is being held prisoner. Before he can find the ledger, the Security Branch begin their raid on the brothel. Dai has lost his chance to find the ledger and gain his freedom. The police take him into custody to stand trial for murder. Jin arrives with the ledger. Reluctantly, the officials agree to honor their deal and set Dai free.
As Longwai and his gang are taken away, the prostitutes are also free to leave, but they have no idea where they should go. Dai brings those who will follow him to his parents’ house. At first shocked, his mother takes on the challenge of helping the girls and working with the vagrant boys, who also came along.
She has a school and home built for anyone who wishes to start a new life. Dai and Mei Yee begin their relationship, and they look forward to the future with hope.