The film starts off in 1937 in a secret underground government laboratory where tests are being performed on the effects of marijuana. Private Miller (Bill Hader) is a subject in a test to decide if marijuana should be illegal. Miller, obviously high, begins to insult the government and his superiors. As a result, the government deems marijuana illegal and quickly "disposes of" Private Miller.
Jumping to the present, Dale Denton (Rogen), a 25 year-old process serving stoner, visits his girlfriend Angie (Amber Heard) at her high school and argues about accepting an invitation to eat dinner with her family. Next, Dale visits his equally stoned dealer, Saul Silver (Franco), to purchase cannabis. Saul offers him a rare strain of marijuana called Pineapple Express. After making his purchase, Dale agrees to smoke a 'cross-shaped' joint with Saul before leaving. He then leaves his apartment, smokes more, and ends up in front of the house of the next person he is supposed to serve, Ted Jones (Gary Cole), who Saul has mentioned as a drug lord.
While waiting outside in his car smoking a joint made of Pineapple Express, Dale witnesses Ted and a crooked police officer, Carol Brazier (Rosie Perez) commit murder. As Dale panics, he throws his roach out the window and leaves the scene, while hitting two parked cars in the process (one being Brazier's police cruiser). As Carol and Ted run outside to find Dale driving away, they determine that he saw the murder and are able to classify the roach he threw out the window as the rare Pineapple Express. Dale drives back to Saul's in a panic to ask Saul about Ted and eventually concludes that he could be traced back to Saul by the rare Pineapple Express (Saul is the only dealer who has it, and Ted is the only supplier). Dale and Saul leave the apartment. Meanwhile, Ted is determined to catch the witness of the murder so he dispatches two henchmen, Matheson (Craig Robinson) and Budlofsky (Kevin Corrigan), to find and dispose of Dale and Saul.
The henchmen go to Saul's middleman, Red (Danny R. McBride) and try to arrange a meeting between Red and Saul, which subsequently fails because Dale and Saul spend the night in the woods. With just the Pineapple Express with them, Dale and Saul head over to Red's apartment where a fight eventually breaks out between the three of them. Dale and Saul are now convinced that the henchmen came to the house and that they must leave the city. Because the henchmen could not kill Dale or Saul, Ted believes that the rival drug gang, led by the "Asians" have hired Dale as a hitman, and Ted goes to war with the Asians.
With no money to buy bus tickets, Dale and Saul (at Dale's suggestion) sell some of the Pineapple Express to some school kids. Saul goes off to buy food and Dale stays behind and smokes more; however, the school's police officer catches him for selling the marijuana to the kids. On a routine ID check, a warrant shows up for the earlier hit-and-run (ostensibly orchestrated by Officer Brazier). Dale tells the female officer about the murder he witnessed and she actually believes him and wants to help. As they drive off, Saul jumps in front of the car, splattering the windshield with a red slushee. The cop gets out to investigate while Saul sneaks into the police cruiser and drives off. Officer Brazier arrives in her car shortly thereafter. A high-speed chase ensues, and Dale and Saul successfully evade her.
Once again, Dale and Saul have a disagreement, with Dale telling Saul he is not his friend, just his drug dealer. Saul takes it really hard and leaves Dale to go to his grandmother's assisted living home. Budlofsky and Matheson are there looking for him, and kidnap and take him hostage to Ted's Pineapple Express grow house (which happens to be the underground base from the beginning of the film, disguised beneath an empty barn); while failing at escaping the two, Saul winds up scarring Matheson's face by pouring scalding hot coffee on him.
Dale has a revelation and, with Red, decides to break Saul out of the grow house, but Red chickens out at the last minute, which results in Dale being held captive. At the same time, the Asians break in and start shooting everyone and a gun-battle ensues. Dale and Saul manage to free themselves despite the fact that the walls are super thin, and everything they've said about escaping, Matheson could hear. Saul escapes through an insulation pipe while Dale fights with Ted. Saul re-enters the barn with a gun yelling, "Fuck the Police" but shoots wildly, completely missing his intended target, Carol. The two engage in hand-to-hand combat which results in Saul defeating Carol but being cornered at gunpoint by Matheson, who has already killed Budlofsky for trying to bail during the shootout. Red suddenly crashes through the barn in his car and saves Saul, and the two reconcile. However Red is then shot multiple times by Carol (making this the fifth time he is shot in the movie) and is believed to be dead. Meanwhile, the last surviving Asian has made it into the underground grow house with a bomb, which he throws down at Ted and Dale. Ted is killed and Dale manages to survive the explosion by taking shelter in a dumpster. The fireball throws Red's car onto Carol, which kills her but knocks out Saul. With Saul unconscious, Dale carries him out of the barn just before it explodes. Seconds after the explosion, Red crawls out, unharmed, and the three go off to a diner and reminisce about the events of the previous day (mentioning the car chase and the fact that Red has lost a lot of blood and should probably see a doctor), as if it were just a crazy night out. They rekindle their friendship and leave to catch a ride with Saul's grandmother. Then the credits roll.