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.
"420" contains examples of:
- 420, Blaze It: The episode is titled "420", and the plot is about the legalization of pot.
- All for Nothing: Carter agrees to publish Brian's book in exchange for Brian completely changing his position on pot and making it illegal again. Brian does so, only for his book to be panned by every critic in America and not sell any copies.
- Are You Pondering What I'm Pondering?: After 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 must make a change if he intends on doing so. It's implied that Lois was going to tell Brian that he should just simply stop smoking marijuana.
- Arson, Murder, and Jaywalking: 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 Doctor Who are through the roof.
- Artistic License – Law: Despite Brian convincing all of Quahog to support cannabis, that would not make it legal at a state level, which is what would need to happen for people in the town to be able to smoke it so openly. The town would most likely be able to decriminalize it.
- Artistic License – Law Enforcement: As a police officer, Joe wouldn't be the one giving Brian a random drug test at his home. That duty would fall on Brian's probation officer. Brian would also be supervised as he urinated (as awkward as that sounds for both parties) to ensure that he won't fake the results of his test, which is exactly what he tries by asking Stewie to pee for him. Also, even if Joe was a probation officer, he would never be assigned to Brian's case due to the conflict of interest that Brian being a family member of Joe's best friend would create.
- Asshole Victim: Carter's goons viciously beat Peter in the same episode that Peter killed Quagmire's cat and tried to cover that up.
- Attention Deficit... Ooh, Shiny!: Peter's next anti-cannabis PSA references Harold & Kumar Go to White Castle, but he gets off topic by talking about Neil Patrick Harris getting cast on How I Met Your Mother and questioning why Future Ted is voiced by Bob Saget when Josh Radnor is already an adult. Carter was nonetheless satisfied.
- Bad "Bad Acting": Carter and Peter's anti-cannabis video features this from both of them.
- Black Comedy Animal Cruelty: Peter tries to shave Quagmire's new cat with a hand razor, but he ends up killing it.
- Blatant Lies: Urging Brian not to take Carter's deal, Stewie says his book is good enough to publish on his own (all while giving a very forced smile and unconvincing line).
- Bowdlerization: During the "Bag of Weed" song, Stewie sings about how Michael Jackson gets enjoyment by having sex with someone like him. On the TV version, Stewie is clothed and standing on the bed Michael Jackson is in. On the DVD version, Stewie is naked and lying next to him.
- Damned By a Fool's Praise: During "A Bag of Weed", the crowd says that The Rocketeer is a movie stupid people rent when they need a thrill.
- Drugs Are Good: The "Bag of Weed" song portrays drugs, specifically marijuana, as the ultimate way of feeling good.
- Epic Fail: Brian's book is a massive flop to the extent that not a single copy (out of the two million printed) were sold.
- Every Man Has His Price: Brian quickly reverses his pro-legalizing marijuana stance when Carter offers to publish his long-gestating novel, in exchange for promoting an anti-cannabis ad.
- Humiliation Conga: Tom Tucker reports that Brian's book set a record as the worst selling novel of all time. Not even one copy was sold, due to the extensive panning from critics. To add further insult to it, Stewie builds a fort out of unsold books in the living room and refers to it as "a house made of mediocrity."
- Hypocritical Humor: Lois opposes marijuana legalization and disapproves of Brian supporting it, despite having smoked pot herself. Brian even calls her out on it.
- Jerkass: Peter, who not only kills Quagmire's cat, but flatly states he killed the cat, grabs the reward money and closes the door on him before Quagmire has a chance to react.
- Kindhearted Cat Lover: Quagmire, to the point of irritating his friends with how much he fawns over his cat.
- No-Holds-Barred Beatdown: Peter is given one before and after getting into Carter's car for no other reason than Carter being a Jerkass.
- Selective Enforcement: The cops blatantly ignore the fact that not only does Peter have a small body bag in the backseat along with a note that says "this is where we'll hide the body," but is covered in blood and driving drunk. But upon discovering Brian in possession of a quarter-ounce of pot, they arrest him at once and throw him in jail.
- Shoot the Shaggy Dog: In the end, pot is made illegal again and Brian's novel (which Carter promised to publish if pot was made illegal) ends up selling no copies whatsoever.
- Song Parody: "A Bag of Weed" is set to "Me Old Bamboo" from Chitty Chitty Bang Bang.
- Stoners Are Funny: As if the title's name wasn't enough.
- Take That!:
- Carter and Peter make an anti-cannabis video showing Adolf Hitler as a pot smoker, but are then sued by Fox News 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 Long List of celebrities he hates. Chris Martin gets mentioned four times. Other celebrities mentioned on the list include: Carlos Mencia, Amy Winehouse, Paul Tsongas, Andy Samberg, Justin Timberlake, 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.
- Technician Versus Performer: Crops up a little during Brian's original protest in the park. He is the technician, as he stands on a stage and loudly addresses the facts of the situation. Every single person there ignores him no matter how loudly he talks. Stewie serves as performer, suggesting doing a fun song to get people invested in his cause. Brian takes his advice, and it works.
- Title Drop:
- Parodied in a cutaway, as Peter loves it when movies do this.Jack Ryan: I'm telling you, these drug dealers represent a Clear and Present Danger to the United States!
Peter: Ah! Ah! He said it! He said it!
Melvin Udall: All I'm saying is, what if this is As Good as It Gets?
Peter: Ah! Ah! There it is! There it is!
Superman: The only way for me to solve this crisis is to be Superman IV: The Quest for Peace.
Peter: So, that's why they called it that. - Later, after Peter and Brian get stopped, the cops find Brian's pot and arrest him.
- Parodied in a cutaway, as Peter loves it when movies do this.
- To the Tune of...: "A Bag O' Weed" is sung to the tune of "Me Ol' Bamboo" from Chitty Chitty Bang Bang.
