The Texas Chainsaw Massacre

On August 18, 1973, five young adults - Erin, her boyfriend Kemper, and their friends Morgan, Andy, and Pepper - are on their way to a Lynyrd Skynyrd concert after returning from Mexico to buy marijuana. While driving through Texas, the group pick up a distraught hitchhiker they see walking in the middle of the road. After trying to talk to the hitchhiker, who speaks incoherently about "a bad man", she pulls out a .357 Magnum and shoots herself in the mouth.
The group goes to a nearby eatery to contact the police where a woman named Luda Mae tells them to meet the sheriff at the mill. Instead of the sheriff, they find a young boy named Jedidiah, who tells them that the sheriff is at home. Erin and Kemper go through the woods to find his house, leaving Morgan, Andy, and Pepper at the mill with the boy. They come across a plantation house and Erin is allowed inside by an amputee named Monty to call for help. When Erin finishes, Monty asks her for help. Kemper goes inside to look for Erin and is killed with a sledgehammer by Thomas Hewitt, also known as "Leatherface", who drags his body into the basement to make a new mask.
Meanwhile, Sheriff Hoyt arrives at the mill and disposes of the hitchhiker's body, wrapping her in cellophane and putting her in his trunk. When Erin returns, she finds that Kemper is still missing. Andy and Erin go back to Monty's house, and Erin distracts him while Andy searches for Kemper. Monty realizes Andy is inside and summons Leatherface, who attacks him with his chainsaw. Erin escapes and heads towards the woods, but Leatherface slices Andy's leg off and carries him to the basement, where he is impaled on a meat hook.
Erin makes it back to the mill, but before she can leave the sheriff shows up. After finding marijuana on the dashboard, he orders Erin, Morgan, and Pepper to get out of the van. The sheriff forces Morgan back into the van, and after giving him the gun he took from the hitchhiker, he tells him to reenact how she killed herself. Morgan, scared and disturbed by the sheriff's demands, attempts to shoot him only to find that the gun is unloaded. Sheriff Hoyt handcuffs Morgan and drives him to the Hewitt house, taking the van's key with him. Erin manages to hot wire the truck but the wheels fall off. Leatherface arrives shortly after and starts hacking through the roof.
When Pepper attempts to run, she is killed by Leatherface. After seeing that Leatherface is wearing Kemper's face as a mask, Erin runs and hides in a nearby trailer belonging to an obese middle-aged woman (known only as 'Tea Lady'), and a younger woman named Henrietta, who offer her tea. The women act strangely, and after telling Erin they don't have a phone, a phone rings and Henrietta tells someone on the other end "she's here". Erin discovers they have kidnapped a child when she sees that the baby with them is the same child in a photograph with the hitchhiker. However, the tea is drugged, and Erin passes out before she can leave the trailer. Erin wakes up at the Hewitt house surrounded by the Hewitt family: Leatherface, his mother Luda Mae, Sheriff Hoyt, Uncle Monty, and Jedidiah. Luda Mae tells Erin that her excuse for her son's actions is that he was tormented his whole life because of a skin disease that left his face disfigured, and she felt no one cared for her family besides themselves. Erin is taken to the basement, where she finds Andy. After several failed attempts to help him off the hook, she stabs him to end his suffering.
Afterwards, she finds Morgan handcuffed in a bathtub. Jedidiah, who does not agree with his family's actions, leads them out of the house. Jedidiah rejects Erin's plea to come with them, and distracts Leatherface long enough for them to escape. Erin and Morgan find an abandoned shack in the woods and barricade themselves inside. Leatherface breaks in and discovers Erin, but Morgan attacks Leatherface, causing him to drop his chainsaw. Leatherface lifts Morgan, entangling his handcuffs in the chandelier, and saws through his groin, killing him. Erin runs out of the shack and escapes through the woods, pursued by Leatherface. She finds a slaughterhouse and hides in a locker. Leatherface opens the locker across from hers and she attacks him with a meat cleaver, chopping off his right arm. Erin runs outside and flags down a trucker, who she tries to convince to drive away from the Hewitt's house, but he stops to find help at the eatery. Erin sees Luda Mae and Sheriff Hoyt talk to the truck driver, while Henrietta watches over the kidnapped baby. When Henrietta walks outside to join Luda Mae and Sheriff Hoyt, Erin sneaks the baby out of the eatery and places her in the sheriff's car. Erin hot wires the car and Hoyt tries to stop her, but she runs him over repeatedly until he is dead. Leatherface suddenly appears in the road and slashes the car with his chainsaw, but Erin manages to escape with the baby, and he watches in anger as she drives away.

Question: What do the five young adults but in Mexico?
Answer: marijuana.