On a Friday morning in the early 1960s, Southern California high school seniors Dave Cartman, Spider Church, and Woody prepare for their school day, to be followed by a long-awaited trip to Tijuana, Mexico. Upon arriving at school, Perlman, the fourth member of the expedition, suddenly withdraws and leaves his friends $95 short of their required funds. Dave’s entrepreneurial younger brother, Wendell, nicknamed “Wimp,” offers to supply the money, but only if he is allowed to go in Perlman’s place. Dave reluctantly agrees to the proposal, hoping Wendell will not interfere as the older boys tour the city’s nightclubs and brothels. The group faces another obstacle when Spider is given an after-school detention for defending himself against an irate football player. However, Spider is undeterred and escapes the campus to joins his friends. Along the way, the boys stop at a grocery store owned by Larry and Kathy, a married couple involved in a heated argument. The boys use the opportunity to shoplift their supplies, unnoticed by the distracted storekeepers. As they race from the parking lot, Kathy requests a ride and announces her plans to file for a Mexican divorce. As the group enters Tijuana, Kathy disembarks at a law office, and the others continue into town. They stop at an automotive upholstery shop, where Dave orders “tuck and roll” vinyl seats for his 1957 Chevrolet. The sheriff appears and expresses admiration for the car, then apprises Dave of a damaged brake light. When Spider antagonizes the sheriff by offering him a $5 bribe, the lawman warns him that their next meeting will not be as cordial. Dave tells Wendell to stay with the car while the other boys make their way to a nightclub and brothel called “The Tunga Lei,” where each of them pairs with a prostitute. Despite plans to lose his virginity on the trip, Woody is unable to perform with the mature prostitute he has chosen, and she attempts to comfort him by offering motherly advice. Later, as Woody and Spider discuss the incident at the bar, Kathy enters, and Spider greets her with a crude proposition. When she and Woody criticize his behavior, Spider announces that his friend is impotent. Woody leaves in humiliation, followed by Kathy, who offers to keep him company. While Spider broods over his altercation with Woody, Dave wanders onto the street and encounters Wendell, carrying a large bag of fireworks that he plans to sell at a profit. Stimulated by his first sexual encounter, Spider is on a mission to procure an aphrodisiac known as “Spanish fly,” which supposedly induces nymphomania in women. Despite Wendell’s insistence that the substance does not exist, Dave requests it from a pharmacist, who threatens him with a rifle. Meanwhile, Woody and Kathy become acquainted, and she compliments him on his desire for romance instead of having meaningless sex. She wishes her husband could be as sensitive. Their mutual attraction grows, stimulated by tequila, and they make love in a motel room. Elsewhere in town, Dave pays a taxicab driver $12 for six doses of Spanish fly, ignoring Wendell’s insistence that the white tablets are common aspirin. At a bar called “The Blue Fox,” Spider triggers a brawl while defending himself against three irate U.S. Marines. The sheriff interrupts the fight and arrests Spider, ignoring the other perpetrators. Meanwhile, Dave dissolves a tablet in the drink of an attractive young street vendor, hoping for another sexual opportunity. While he waits for the drug to take effect, a local boy named Pablo parks his customized Ford Model A nearby, and Dave initiates a conversation with him, unaware that Pablo is the street vendor’s brother. When Dave reveals his intentions toward the girl, Pablo and his friends force Dave and Wendell into the car, and drive them to an auto salvage yard. As Woody and Kathy leave the motel, they see Spider in the back seat of the sheriff’s car and follow it to the police station. Spider is placed in a holding cell, where a large prisoner challenges him to a fight, then overpowers the boy and knocks him unconscious. Woody and Kathy try to reason with the sheriff, but he demands $3,000 to release Spider. At the salvage yard, Dave is hoisted on a crane while Pablo threatens to remove his genitals with a blowtorch. Unaware that his brother is being subjected to a mere prank, Wendell commandeers Pablo’s car and comes to Dave’s rescue. The Cartman brothers arrive at the upholstery shop, where Spider, Woody, Kathy, and the sheriff are waiting for them. Woody informs Dave of Spider’s arrest, and explains that they were forced to offer the Chevrolet in exchange for their friend’s release. As Dave and the sheriff argue over the exchange, Spider and Woody knock the lawman to the ground and handcuff him. Wendell ignites a load of fireworks in the police car and the group makes its escape. Moments later, Pablo and his companions reclaim the Ford and give chase, until Wendell disables the car by firing a skyrocket into its radiator. The group reaches the border to find a slow-moving line of cars in front of them and the sheriff approaching from behind. Spider misinforms a border guard that the Chevrolet is carrying illegal contraband, and the car is towed to the front of the line, away from the sheriff’s reach. A thorough investigation reveals nothing but the sock in Dave’s underwear, and as Kathy and Woody wait for the car to be reassembled, Larry appears, begging his wife to return. She bids Woody an affectionate farewell and leaves with her husband. As the sun rises, the boys stop at a diner on a coastal highway, where they encounter the same group of Marines, and another brawl ensues.