During a pep rally at South Park Elementary, a local PETA group protests the use of a cow as their mascot. The school agrees to pick a new mascot and the students are told to vote for a new mascot. Embarrassed by the bland choices, the kids decide to fill in a joke candidate, but disagree as to whether it should be a "giant douche" or a "turd sandwich." Kyle rallies his friends to fill in the giant douche and Cartman gathers support for the turd sandwich. Kenny joins Kyle, possibly because Cartman had recently broken his cat's leg, and Cartman wins Butters' support (by slanting the question when asking Butters which of the two choices was funnier). The result is a tie between the two joke candidates and a runoff election is to be held. The turd sandwich supporters rally against the giant douche supporters, using bribes and scare tactics to gain support.
Stan decides that he doesn't like either candidate, declaring that he won't vote at all, much to everyone's dismay. Stan is equally dismayed, wondering why anyone would care about such a pointless exercise. He talks about it with his parents, who are not impressed at all with his decision and insist he does vote, whilst then getting into a vote over who should win the election once they find out who the candidates are. Stan's decision not to vote angers the entire town, and various attempts are made to make him change his mind, including having P. Diddy terrorize him with a literal Vote or Die campaign. Not wanting to die Stan concedes to voting. However, when he tries to vote for Turd Sandwich Kyle refuses to let him and demands that he change his vote to Giant Douche. Realizing his best friend was just using him to beat Cartman, Stan once more refuses to vote. The school administration decides that Stan must be banished from the town for all eternity (or until he decides that voting is important). He's placed on horseback, bound with a bucket over his head, and sent off into the woods.
Eventually, he reaches the PETA encampment, where he finds that the humans have grown intimate with the animals to the point of bestiality. The head human member gives Stan what he needed the whole time: a proper reason why he should vote rather than 'because you have to'. They tell him that an election is always between the giant douche and a turd sandwich, because they're the only people who suck up enough to make it that far in politics, which Stan understands. P. Diddy shows up at the camp to kill Stan, but is distracted by a PETA member who throws a bucket of blood on his fur coat. This angers him and he takes his rage out on the PETA camp, killing all the members. Stan gets shot in the shoulder, but manages to escape and flee the camp.
Returning to South Park, Stan is finally convinced to vote and chooses the turd sandwich. Despite adding his vote to the tally, the giant douche still wins the vote 1410 to 36. Stan is initially dismayed that his vote didn't matter, but Randy informs his son that all votes matter even if it's for the losing side. Suddenly, Mr. Garrison runs in with the news that the PETA members have all been discovered murdered. As a result, the cow is reinstated as South Park Elementary School's mascot. Randy tells Stan that now his vote didn't matter, much to his annoyance.
The credits are played to a takeoff of the Rock the Vote theme, suitably reworded:
Let's get out and vote!
Let's make our voices heard.
We've been given the right to choose,
between a douche and a turd.
It’s democracy in action!
Put your freedom to the test.
A big fat turd or a stupid douche,
Which do you like best?