If you haven't heard of 'Serial' yet, you're probably living under a rock. The weekly podcast follows reporter Sarah Koenig as she investigates a 1999 murder; the show slowly unravels the key details of the case, hooking listeners into hours of binge-listening or, if they're caught up, a long, anxious wait for the next episode. The formula is working: 'Serial' is currently the No. 1 podcast on iTunes. Coworkers are posting spoilers on Facebook; Slate has created a new podcast dedicated to unspooling it (yes -- a podcast about a podcast); there's an entire subreddit dedicated to discussing its finer points.

But you might not know that Nick Thorburn of the Unicorns and Islands composed the music for the podcast -- including that bright-yet-melancholy title theme, 'Bad Dream,' which is as much a part of the identity of the show as Hae Min Lee's hopelessly-teenage diary entries or host Sarah Koenig's incessantly-interrupted phone calls with Adnan Syed.

Thorburn composed the music at home, between touring for Islands and the Unicorns, who played a series of shows opening for Arcade Fire this summer. Thorburn says it took him most of a single weekend, spent listening to the first episode of the show and tinkering with sounds on his OP-1 synthesizer. (He finished the project while he was on tour with Islands; 'Serial's' producer Mark Henry Phillips also composed some of the music.)

Thorburn says working on commission was a change of pace. "With Islands, it's a much more casual flow," he said. "I pick up the guitar if the mood strikes me, or if a melody comes into my head. This was: 'I'm going to chisel away something that will go into [the podcast].'"

Thorburn got involved with 'Serial' through a friend of a friend who worked on music for 'This American Life.' ('Serial' is a 'TAL' spinoff.) Once the show began airing in mid-September, Thorburn started seeing feedback almost immediately -- in person and on Twitter. "I only retweet the mean stuff," says Thorburn with a laugh. "No -- every day people say the kindest stuff. And people come up to me all the time in public places. If I'm out in a social setting, people talk about it. It's funny."

Some on Twitter -- including one person Thorburn retweeted -- thought the song sounded a lot like 'One (Is the Loneliest Number).' "I mean, because it does sound like the [Harry] Nilsson song, with the plunking piano line, descending melody, you know. Though I think the differences are strong enough that I'm not going to get sued."

Thorburn himself is a fan, and says he listens to the show every Thursday as new episodes are released. Though unlike most listeners, he's had a hand in one of the key elements of the show. "It's distracting," he says. "Sometimes I'll lose the plot because I'm distracted -- my song will come on. 'Oh, they used it there!' I do sometimes lose the storyline. But sometimes I don't even think about it -- it just happens to be there. It's weird."

Any 'Serial' spoilers he could share? "He did it," Thorburn said, with a laugh.

Listen to Thorburn's music for 'Serial,' including 'Bad Dream,' below: