The Narcotic Division of the Los Angeles Police Department, headed by Captain Hayes, battles the evil menace of marihuana, which they characterize as a killer because heroin, cocaine and opium addictions follow upon its use. After Markey, a notorious dope dealer, sells marihuana cigarettes to several teenagers, they smoke them and become involved in a car crash in which three are killed and one girl loses both of her legs. Hayes assigns lieutenants Tyne and Mason to go undercover and pose as a high school student and bartender, respectively, in order to find the peddler. At the club St. Pierre, Markey sells marihuana to Rita, a chorus girl, and expresses an interest in a new girl, Ann Lester, telling Rita to arrange to have Ann come to a party. As Ann is expecting her brother Bob home on a visit from college, she agrees to hold the party at her house. During the party, Markey makes a pass at Ann and gives her a marihuana cigarette. After a few puffs, she falls prey to his dubious charms. When Bob gets home, having missed the party, he finds the house in a mess and a man passed out on the sofa. Upstairs, Ann is asleep. Later, Bob tells Ann that he feels guilty that she is paying his way through college, but she wants to continue and has realized that she can earn more money by becoming involved in Markey's operations. One day, while Markey is visiting his supplier, Jonathan Treanor, Treanor receives a phone call informing him that a load of marihuana has been hijacked on its way into California from Arizona. He suspects one of his men of double-crossing him and instructs his henchman, Raymond, to kill him. Meanwhile, Ann becomes involved in a cycle of parties, smoking and drinking, while earning lots of money. At one party, a pianist plays "Chopsticks," but hallucinates that he is performing a classical piece at the Hollywood Bowl. Another man, under the influence of the weed, thinks that he is dying and thrusts his hand through window panes. Eventually, Hugo, the club's boss, fires Ann and Rita because their dancing has greatly deteriorated. One night, while Ann is selling marihuana at a party and saying that it is financing her brother's education, Bob comes in and is horrified by her behavior. The next day, Ann discovers that Bob has hanged himself in the garage. Hugo decides to go to the police and tells them about Ann's involvement with Markey, whom Mason and Tyne then place under constant surveillance. Ann has gone to pieces following Bob's death and is arrested by Mason and Tyne when they break up another marihuana party at her house. Captain Hayes interrogates Ann, but she denies knowing Markey and his associates, so he introduces her to three prisoners who are severely addicted. Hayes and Ann also visit the morgue and view a body, a suicide resulting from addiction. However, Ann does not break and still refuses to talk. Sentenced to serve sixty days in the County Jail, she suffers great remorse for having caused Bob's death and experiences hallucinations. Realizing that none of her friends have come to visit her, she is cool toward Markey when she is released and decides to cooperate with the police. After she tells Markey that she has been approached by the head of another dope syndicate, Markey and Treanor request to meet him. Posing as the syndicate boss, Mason offers Treanor $75,000 to deliver some "merchandise," but as Treanor does not have enough marihuana on hand, Mason agrees to wait. Later, while Markey and Ann drive to Arizona to pick up more marihuana, Mason waits with Treanor for them to return. However, Treanor has checked on Mason's story and, aware that he is a police officer, holds him at gunpoint. When Markey slaps Ann, Mason jumps Treanor, then goes after Markey. Police reinforcements arrive and Treanor and Markey are arrested. Later, Hayes thanks Ann for her cooperation, and she, in turn, thanks him, Mason and Tyne.