In New York City, soft-spoken Helen returns to the apartment she shares with her boyfriend Marco after enduring an unhygienic and inept abortion. Although Helen becomes ill from the procedure, Marco is satisfied because he was able to procure the abortion through bartering, and thus did not have to expend any money. When Marco leaves briefly to collect some cash, Bobby, an amiable small-time drug dealer to whom Marco owes money, shows unexpected gentleness and concern for Helen. Soon after, when Helen is hospitalized for profuse bleeding, Bobby sneaks in after visiting hours and tells her that Marco has gone away. Briefly, Helen considers returning to her dysfunctional family in Fort Wayne, Indiana. However, after she is released from the hospital, she moves in with Bobby. As they are walking together, Bobby steals a television out of a parked van and pawns it for money to buy drugs. At his apartment, Bobby declines Helen's invitation to have sex until she feels better. When she awakens to find him taking drugs, he explains that he is not an addict, but only "chipping."At Sherman Square, which has been nicknamed "Needle Park" because it is a hangout for drug dealers and addicts, Bobby introduces Helen to various acquaintances that make up his social group of drug customers. At a cheap luncheonette, she meets Bobby's older brother Hank, who wears a suit and burgles for a living. In an apartment used by several addicts, Helen witnesses the intricate ritual of "shooting up" or preparing and injecting the heroin into a vein, and listens to their discussion of the price and availability of drugs. On the street, Bobby becomes anxious when drugs he has paid for are not in the telephone booth where he is to pick them up. After tracking down the man who sold him the drugs, he is re-directed to look into a trash can. That night his drug-taking renders him unable to make love to Helen.When they are evicted for non-payment of rent, Bobby suggests to Helen that she return home, but she refuses and they move into a sleazier apartment. After Bobby asks her to deliver money to Freddy, one of his suppliers, Helen realizes Bobby has not given her enough money to pay for the drugs and that she will be expected to make up the difference by having sex. While Helen is negotiating with Freddy, Hotch, a policeman on the drug squad, and his partner arrest them. As his partner convinces Freddy to assist them in a sting, Hotch explains to Helen about what it's like when there is a "panic." He explains that when the drug supply on the street is low, everyone begins to turn one another into the police, in return for favors. Unexpectedly, Hotch releases Helen without booking her and she returns to Bobby, who begins to use drugs more heavily. While Bobby spends more time sleeping, Helen, who all along has only observed the others, begins to shoot up.Some time later, Bobby is exuberantly playing stick ball with some children, when he passionately kisses Helen and looking into her eyes, realizes she is now using. In his favorite diner, as the drugged Helen sits listlessly beside him, Bobby proposes. Upon hearing the news, Hank asks what they will live on and doubts Bobby's supposition that he can quit drugs. Hank offers Bobby work as a burglar, but Helen rejects the idea, saying instead that she will get a job. However, Helen who has been hired as a waitress, does a terrible job and soon quits. Just before Bobby is to assist Hank in a burglary, he overdoses. With the aid of other users, Helen finds Bobby and helps him through the ordeal, but Hank is angry with Bobby for jeopardizing his plans. Reluctantly, he allows Bobby to assist him on another night, but the theft goes awry and Bobby is arrested. While he is in jail, Helen finds it harder to get drugs and has sex with Hank, in exchange for heroin.When Bobby is released, he and Helen have a big fight, after which Bobby considers moving to the country, but they agree that it is not feasible. Bobby persuades Santo, a major drug dealer, to let him handle distribution in Needle Park, and is directed to an apartment where the heroin is being prepared and bagged. Meanwhile, as they are in need of immediate money, Helen turns to streetwalking. After Bobby distributes Santo's drugs, Needle Park residents are happy to have a reliable source for a while. Bobby tells Helen that he stashed some drugs away as an insurance in case he is arrested, so that he can pay for a lawyer. However, when he finds the drugs missing from his hiding place, he accuses Helen of stealing it. As her health deteriorates from increasing drug use, their relationship suffers. Hotch, who is not surprised to watch her fall from an innocent woman in love to a drug abusing prostitute, keeps an eye on her and, when she is arrested with her john, he asks the arresting officer not to book her, as he needs her for something he is planning.When her mother writes, asking Helen to meet friends who are visiting the city, Helen is reluctant, but dresses up, carefully trying to hide the track marks on her arms. Instead of meeting them, however, she picks up a young virginal customer. When Bobby finds her, he scares the boy away, but then he and Helen begin to laugh. Realizing they have been too serious, they take the ferry to the countryside, where they buy a puppy. On the return trip, they discuss making a fresh start and Helen suggests that they move out of Needle Park, but Bobby refuses and convinces her to go into the men's room to shoot up. When the dog begins to whine, he puts it outside the door. Afterward, Helen discovers the dog missing and finds it just before it falls off the end of the ferry and into its machinery.Desolate, she goes to see Marco, who is back from his trip, but soon returns to Bobby and steals drugs from him. Needing a "fix," she goes to a doctor, falsely claiming she needs drugs for a painful kidney stone. Aware that she is an abuser, the doctor refuses to write a prescription for her, but gives her some samples, telling her never to return. She takes some of the pills, but is arrested for selling the rest to minors. Hotch warns her about the dangers of the women's prison and knowing that Bobby can lead them to Santo, offers to arrange for the charges to be dropped if she will help them to catch Bobby in the act of picking up a drug shipment. When she refuses, Hotch explains that she will go to jail for one to three years, but Bobby, who knows how to work the system, will probably only serve six months. Later, at home, Bobby elatedly tells her that there will be no more "panic" after the new shipment arrives, saying that Santo is giving him as much as he can sell, and is allowing him to have people work under him.In the next two weeks, Hotch approaches Helen several times, reminding her of the pending trial. Depressed, she increases her drug use and spends days in bed, until even Bobby is annoyed. Helen finally agrees to help the police and late one night Helen and Hotch watch, as a squad of policemen apprehend Bobby, who is in possession of a large quantity of heroin. Spotting her on the street and feeling betrayed, Bobby yells, "I was gonna marry you!" Months later, when he is released, Helen waits for him at the gate. Although his first impulse is to rebuff her, he calls to her and they walk off together.