As Anthony "Man" Stoner, portrayed by Cheech Marin, navigates the pressures of life with his parents' ultimatum—find a job by sundown or face military school—he bravely embraces the journey ahead. After finding himself stranded on the highway, fate smiles upon him when he is picked up by Pedro de Pacas, brought to life by Tommy Chong, an equally enthusiastic stoner. Their adventure kicks off with a shared massive joint, only to make a comical discovery that it was tainted by dog droppings after a dog munches on Man's stash.
As they ride the high, an anxious Pedro struggles to breathe, leading to an unintended LSD encounter facilitated by Man. Their psychedelic journey is abruptly interrupted when law enforcement finds them slumped in their vehicle on a traffic median. What follows is a humorous trial that skewers the traditional courtroom drama, where Man cleverly exploits the judge’s vodka-fueled state to secure their release due to a technicality.
To replenish their dwindling marijuana supply, the duo visits Pedro's cousin Strawberry, a Vietnam War vet who revels in celebrations. However, chaos erupts when someone mistakenly snorts lines that Man had set up as cocaine, causing a ruckus just as the police raid the party. Strawberry’s confusion between officers and Viet Cong during a flashback leads the group, including Pedro's family, to a comedic deportation to Tijuana as they seek an easy ride to a wedding.
Amidst their zany endeavors to retrieve a car from Pedro's uncle, they unwittingly get involved in smuggling a van made entirely of "fiberweed," a THC-heavy resin, as the hapless Sgt. Stedenko and his narcotics unit pursue them. A series of comic misfortunes follow during their attempts to cross the border, culminating when a group of unsuspecting nuns divert attention away from these mismatched smugglers.
As their trip takes multiple twists, their van becomes the target of Stedenko's pursuit, but luck strikes when a tire is punctured by one of his own officers, halting the chase. They eventually pick up two hitchhiking women who sway them to perform at a Battle of the Bands at the revered Roxy Theatre. With Man's band, Alice Bowie, on the brink of success, they narrowly escape arrest when a motorcyclist is unintentionally impaired by fumes emanating from their "fiberweed" van.
When they hit the stage, one of the women mistakenly provides Man with a powerful substance she believes to be an "upper," sending him into a frenzy. Despite the chaos, Alice Bowie captivates the audience—including the stoned police officers, thanks to the cloud of marijuana smoke wafting from their van.
With triumph on the horizon, Pedro and Man embrace their newfound success and set off on a road trip in Pedro's car, daydreaming about what lies ahead in their careers. Man lights a bit of hashish, sharing it with Pedro as they drive into the sunset, leaving a trail of euphoric haze and the melodies of their journey echoing into the distance.
