Best friends and roommates Jesse Montgomery III and Chester Greenburg awaken with hangovers and no memory of the previous night. The answering machine contains an angry message from their twin girlfriends, Wilma and Wanda, as to their whereabouts. The two also learn that they have almost been fired from their pizza delivery jobs, and they discover that the cabinets in their kitchen are full of containers of pudding. They emerge from their home to find Jesse's car missing (leading to the titular question) and, with it, their girlfriends' first anniversary presents. Because the girls have promised them a "special treat", which Jesse and Chester take to mean sexual intercourse, the boys desperately begin retracing their steps in an attempt to discover where they left the car. Along the way, they encounter a
transgender stripper, a belligerent speaker box operator at a Chinese restaurant's drive-through, two tattoos they discover on each other's backs, UFO cultists led by Zoltan (who later hold the twins hostage), a
Cantonese-speaking Chinese tailor, the
Zen-minded Nelson and his
cannabis-loving dog Jackal, beautiful Christie Boner, her aggressive
jock boyfriend Tommy and his friends, two hard-nosed police detectives, and a reclusive French
ostrich farmer named Pierre. They also encounter two groups of aliens: one group consists of five attractive women, and the other comprises two Norwegian men, all of whom are searching for the "Continuum Transfunctioner" —an extraterrestrial device that Jesse and Chester accidentally acquired the night before. The search for both the car and the Transfunctioner leads Jesse and Chester to the local arcade and mini-golf course, Captain Stu's Space-O-Rama. Once inside, they encounter Zoltan and his cultists, who give them Wilma and Wanda in exchange for a toy that Jesse and Chester try to pass off as the Transfunctioner. Tommy, Christie, and the jocks arrive along with Nelson and his dog, whom they release after Tommy snatches the fake Transfunctioner from Zoltan. The two sets of aliens arrive and reveal that the toy is not the Transfunctioner; moments later, Chester finishes solving a
Rubik's Cube, which shapeshifts into the real Continuum Transfunctioner. The boys are warned that once the five lights on the device stop flashing, the universe will be destroyed. Jesse and Chester must determine which group of aliens is there to protect the universe and which is there to destroy it. Both claim to be the protectors of the universe, stating that they were with Jesse and Chester the previous night, which Jesse and Chester still cannot remember. The two correctly choose the Norwegian men after they successfully answer their question about the previous night by stating they got a hole-in-one at the 18th hole at the arcade's
miniature golf park and won a lifetime supply of pudding. At the last second, they deactivate the Transfunctioner, saving the universe. Enraged, the five alien women merge into a beautiful 50-foot
giantess clad in a purple bra and matching miniskirt. She swallows Tommy whole in front of Christie (who reacts with indifference), and gives chase to Jesse and Chester. The cultists tell them to activate the Photon Accelerator Annihilation Beam on the Transfunctioner. However, the button that activates it is too far in to reach. Chester suddenly remembers a nature show from earlier in the day, with
chimpanzees using sticks to gather food through small holes, and applies the same logic with a straw to push the recessed button, destroying the alien. Tommy survives, but Christie breaks up with him in favor of Nelson. The protectors thank Jesse, Chester, and the twins for saving the world, and erase their memories of the adventure. The protectors park the duo's car, a
Renault Le Car, behind a mail truck for them to find the following morning. Jesse and Chester salvage their relationships with the twins and discover that the special treat from the girls turns out to be matching berets with their names, Jesse's and Chester's, embroidered in the front. The protectors leave a gift for their girlfriends, and for them by extension:
breast enhancement necklaces. Jesse, Chester, and the twins go out for Chinese food in Jesse's car while arguing about what their tattoos say. ==Cast==