Quagmire's personality changes when he adopts a pet cat, which irks Peter. Meanwhile, after Brian gets arrested for having a bag of weed in his car (during Peter's botched attempt at ditching Quagmire's dead cat in the woods), he starts a campaign to have marijuana legalized.
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!!"Episode 420" contains examples of:
* AreYouPonderingWhatImPondering: When Lois catches Brian trying to use some of Stewie's urine to use in a random drug test orchestrated by Joe, Lois scolds Brian for going to such lengths and tells him that he has to make a change if he intends on doing so--and it's implied that Lois was going to tell Brian that he should just simply stop smoking marijuana.
* ArsonMurderAndJaywalking: Brian says that with pot legalized, crime is down, productivity is up (which, according to DVD commentary, they couldn't say as the censors did not want marijuana to be glamorized), and ratings for ''Series/DoctorWho'' are through the roof.
* {{Jerkass}}: Peter, who not only kills Quagmire's cat, but calmly takes Quagmire's reward money, admits he killed the cat, and closes the door on him before Quagmire has a chance to react.
* SelectiveEnforcement: The cops don't mind Peter with a bloody trashbag or a note saying "This is where we'll hide the body", but go nuts when they catch Brian with a small amount of pot.
* ShootTheShaggyDog
* StonersAreFunny: As if the title's name wasn't enough.
* TakeThat: Carter and Peter make an anti-cannabis video showing UsefulNotes/AdolfHitler as a pot smoker, but are then sued by Creator/{{Fox News|Channel}} for using their copyrighted image of Hitler.
** At one point Peter is too stoned to properly setup his own cutaway, and thus it ends up simply being a LongList of celebrities he hates. [[{{Coldplay}} Chris Martin]] gets mentioned four times. Other celebrities mentioned on the list include: Carlos Mencia, AmyWinehouse, Paul Tsongas, Andy Samberg, JustinTimberlake, Eve Plumb, David Arquette, that forehead guy on ''The Office'' (Rainn Wilson), and Kate Beckinsale. According to DVD commentary, the celebrities on that list were picked at random and the writers have nothing against them, though some of the celebrities on that list are considered annoying, detestable, marginally-talented, and/or overrated enough for viewers in real life to hate them.
* ToTheTuneOf: "A Bag O' Weed" is sung to the tune of "Me Ol' Bamboo" from ''Film/ChittyChittyBangBang''.