Late one night in Los Angeles, Sgt. Fred Matthews, [Frank Harding], and Officer Lynn Donahue, [Slate Harlow], arrest a pusher who is carrying two pounds of uncut heroin. They are ambushed by gangsters Mitch Swardurski, [Herman Rudin], and Lenny Potter, [Philip Mansour], who kill Matthews and wound Donahue, then kill the pusher after he tosses the briefcase containing the heroin into the underbrush.
The next day, eighteen-year-old Julian "Ves" Vespucci, [Jonathan Haze], delivering groceries from his father's store, teams up with Jim Bowers, [Yale Wexler], a would-be artist, and Nick Raymond, [Steven Marlo], a bodybuilder, to investigate the case. They discover it contains samples labeled "face powder" and end up discarding the canister, though Jim keeps some powder for his girlfriend, Kathy, [Abby Dalton]. The trio pawns the briefcase; Jim takes the samples to Kathy and proposes to her, while she fears he won't be able to support them.
Jim reads a newspaper headline about the missing narcotics and hurries to tell Nick and Ves, but the canister has already been picked up by a garbage truck. After a frantic search at the city dump, they locate it. Nick convinces Jim and Ves to meet Danny, [Allen Kramer], a heroin addict, who is thrilled by the tiny "test" packet and agrees to sell it for them.
The boys are astonished by the money the heroin fetches. Nick and Ves go shopping, while Jim buys a bracelet for Kathy. When he explains where the money came from, Kathy rebukes him for profiting from others' weakness.
Danny describes, in harrowing detail, how addiction tore through his life, and Jim grows increasingly reluctant to continue. The police gain their first break when the pawnbroker reports buying the briefcase and recalls that one of the buyers was Nick, who worked in a garage. Mitch and Lenny learn from a local pusher that Danny has been selling heroin, pushing Jim toward turning the drugs over to the authorities.
Jim decides to abandon the scheme and hand the drugs to the police. Danny is brutally questioned by Lenny and Mitch at his shack; Nick collects the day's earnings but is beaten. Ves is captured and forced to call Jim. Jim retrieves the canister and Ves calls the police.
As Ves calls, Jim climbs a tower in a power plant to escape. Mitch follows, but Jim pours the heroin onto Mitch's face. The police arrive, capture Lenny, and shoot Mitch, who falls to his death. Jim and Ves are arrested, confronted with the consequences of their greed, and left to face what they have become.
