As a result of just how many characters the show has, The Simpsons has featured a number of episodes focused on the backstories of various Springfielders. In the interest of variety, this also means that sometimes those full histories can be a bit surprising, such as the unexpected origins of Homer's friend Carl Carlson, who is actually Icelandic.
The history of The Simpsons has been remarkably fluid over the years, with multiple background characters especially being given unique and unexpected histories. One of the most frequently altered is Carl, who works with Homer at the Nuclear Power Plant. A frequent barfly at Moe's and one of Homer's best friends, some episodes suggest that Carl has been in Springfield his entire life and been pals with Homer and Lenny for years. Other episodes, though, have referenced a more unique backstory for the character, with Season 14's "'Scuse Me While I Miss The Sky" suggesting he'd actually grown up in Iceland. This part of Carl's backstory ended up coming to the forefront in The Simpsons Season 24's "The Saga of Carl."
In the episode, Homer, Moe, Lenny and Carl win $200,000 in the Springfield lottery. However, Carl quickly takes all the winnings and disappears, prompting the other three men to try and uncover the truth of what happened. It turns out Carl is, in fact, originally from Iceland, where he was raised by his adopted parents.
The Carlsons also turn out to be an infamous family in Icelandic culture, as their ancestors are reviled by the public for their part in allowing invading Vikings to ravage the local lands. Carl stole the money to buy a lost piece of a historical tapestry, which he believes will exonerate his family. The other three men steal the tapestry piece from Carl in revenge, only to discover that the tapestry actually not only confirms the Carlsons' treachery but reveals it was far worse than anyone knew.
Taking pity on their friend, the three end up speaking to the other citizens of Iceland, arguing that Carl has proven to be a good enough man to warrant the family name being at least partially redeemed. This turns opinion about the Carlsons around almost instantly, and Carl is able to introduce his parents to the three. Carl then tearfully embraces Homer, Lenny and Moe, admitting that he was wrong to think they weren't his true friends. Carl ends up returning to Springfield alongside them, and the group go back to their lives, with Homer even throwing a party for everyone.
"The Saga of Carl" is a surprisingly in-depth exploration of the titular character and his place within The Simpsons. In terms of theme, the episode largely focuses on the idea of male friendship, and whether just hanging out and doing "guy stuff" actually constitutes a friendship. In his more frustrating moments before reconciling with the group, Carl points out that none of them were aware of his heritage, because none of them open up to each other. Still, Carl's love for his adopted homeland is a fun minor beat for the character and helps flesh out someone often just used for snarky comments or quick punchlines.