In Cambridge, Massachusetts, bored Harvard law student Peter Harkness considers making a marijuana run for his acquaintance, theater instructor and dealer John Hartnup. Despite his board exams scheduled for the following day, Peter meets with John and his girl friend Sandra to offer to pick up eight "bricks" of marijuana in Berkeley, California and return it to Boston. After Peter cavalierly skips his exams, his conservative girl friend and fellow law student Annie learns that he is working for John and threatens to report him to the police. The following day, Peter arrives in the San Francisco Bay area with a special suitcase designed by John then travels to Berkeley where, at the appointed house, he is met by Susan Kunen and dealers Rupert, Musty and Emir. Uneasy about having to wait the entire day before his return flight, Peter asks where to store the suitcase safely and Susan offers to take him to the train station. After placing the suitcase in a locker there, Susan suggests showing Peter around and later, after sniffing some cocaine, the couple has sex in an empty recording studio. Upon Peter's safe return to Boston, he chafes about settling back down with the conventional Annie, but when he attempts to pick up a girl in Cambridge, he is mugged by her associates. Smitten with Susan, Peter pleads with John to allow her to make the next marijuana delivery so that she can come to Boston. Although wary of Susan's inexperience, John agrees, but when Peter phones Susan with the offer, she only reluctantly accepts. On the day of her arrival, Peter waits anxiously, only to learn from John that Susan has telephoned from the airport that her bag was lost in transit and she is reporting its loss. Horrified that Susan is placing herself at great risk of being discovered, Peter demands that John help her, but the dealer angrily reveals that Susan ignored all his detailed instructions about making the trip, which included not checking the bags. To Peter's further dismay, John reveals that Susan was carrying not one, but two, bags with a total of forty bricks, or $7,200 dollars worth of marijuana. Peter races to the airport on his motorcycle, arriving just as undercover narcotics detective Lt. Edward Murphy arrests Susan. On returning to John's apartment, Peter finds him hastily packing in fear of discovery. Settling at Sandra's apartment, John and Peter watch the news, and John is startled to hear Murphy tell a reporter that he has made a seizure of twenty bricks of marijuana. Suspecting that Murphy is corrupt and selling the other half of the drugs on the side, John talks Peter into joining him that evening to follow Murphy. After staking out Murphy's home for several hours, the men see him leave carrying a bag and follow him to a warehouse, where he meets with a Cuban dealer named Cortez. After Murphy departs, John convinces the nervous Peter to hold Cortez up with the only gun he has, a flare pistol, and the pair retrieves the marijuana. In addition, John playfully steals the cigar-smoking Cortez's single humidor. Later, back at Sandra's, John is startled to find Peter on the telephone with Murphy, trying to arrange an exchange of the drugs for Susan. As Murphy rejects the proposal and hangs up on Peter, John discovers that the humidor is full of heroin. Peter excitedly tells John they now have a tool to lure Murphy into freeing Susan, but John refuses involvement, insisting that he only traffics in marijuana. Stubbornly proceeding on his own, Peter takes the humidor and informs Murphy of its contents. Setting up an elaborate process that will allow Murphy to sample a small amount of the heroin to validate it while he remains safely hidden, Peter again demands Susan's release. Murphy agrees and, following Peter's instructions, brings Susan to the city train station. There, Peter instructs him by telephone to leave Susan, then take a cab to Walden Pond, where he can retrieve the heroin. As Murphy departs in a cab that Peter has previously arranged, Peter greets Susan, who acknowledges that all charges against her have been dropped. The couple then hastens to Peter's motorbike where they race off to reach Walden Pond before Murphy. Leaving the heroin container in the snow with a marker, Peter and Susan hide behind a building to observe the pickup. Unknown to everyone, Cortez and two of his men have been following Murphy, and when the detective retrieves the heroin, they run him down with their car. Meanwhile, Murphy's backup detectives arrive as the Cubans are making their escape. After a fierce gunfight, everyone but Cortez is killed, but as the dealer walks away with the remains of the heroin, a revived Murphy shoots him down, then reports the confrontation on the police radio and drives away to a hospital. Stunned by the events, Peter and Susan come out from hiding, look at the heroin clutched in Cortez's hand, then ride away together.