It discusses the series' Running Gag about Lincoln reading comics in his underwear, and its theme is that it's impolite to do it in front of his sisters, but he can still do it alone.
In the story, Lincoln is reading in his underwear, when Luan comes in and wants to read too, but she strips down to her underwear, embarrassing him. She then reveals that she did it to point out the ludicrousness of his behaviour and he apologises.
Sunday Skivvies provides examples of
- Actually Pretty Funny: Lincoln chuckles when Luan reveals that her stripping was to make him look at himself and as her way of asking him to just only read his comics half-naked while in his bedroom.
- Alliterative Title: Sunday Skivvies.
- Baby Talk: When Luan teases Lincoln for blushing, she calls him "Linky Winky" and speaks to him in third person.
- Batman Can Breathe in Space: Apparently the comic book character Muscle Fish can breathe in space, and Luan wants to know how.
- BrotherSister Incest: Joked about when Luan jokingly asks if Lincoln finds her "cute" in her underwear and that's why he's blushing. Lincoln, however, is grossed out by this joke.
- Comedic Underwear Exposure: Reconstructed. This fanfic has Luan point out (through demonstration) that it's rude to wear nothing but your undies in front of your sisters, but he can still do it in private. Luan also admits that she personally never had much issue with Lincoln reading his comics half-naked, she just wanted him to do it less around the house.
- Crush Blush: Discussed when Luan sees Lincoln blushing and asks him if he thinks she looks cute in her undies. Thankfully, he's just embarrassed— and, of course, Luan making that joke only embarrasses him further!
- Don't Explain the Joke: Discussed when Luan tells Lincoln that one of the golden rules of comedy is never to explain your jokes. But she makes an exception in this case so Lincoln can get the point she was making by stripping down to her underwear.
- Double Standard: Defied. Lincoln wants to say that reading comics in his undies is fine, but Luan reading in hers is not because he's a boy and she's a girl, but then he decides that'd be a cop-out and make him seem sexist.
- "Knock Knock" Joke: Discussed when Luan says that comedy is "more than just silly puns and knock-knock jokes".
- Pun: Luan refers to Lincoln's reading comics half-naked as "a comic strip".
- Vulgar Humour: Luan tells a joke about Lincoln blushing because she thinks he's cute.