The episode opens with Sheldon recording a self-journal about his "descent into madness". Sheldon is apparently so far off his rocker, even his precious toilet schedule is thrown off. After he gets up to go to the bathroom, we flash back two days to happier, less crazy times. Sheldon and Leonard are working on physics equations in the living room, while Sheldon hums a ditty out loud. When Leonard points out that Sheldon's humming, Sheldon denies it and says it's probably just his brain "making noises" as it works. When Sheldon realizes he actually is humming, he and Leonard try to figure out what song he actually is humming, to no avail. Leonard asks if Sheldon is going to obsess over the song until he figures out where it's from, to which Sheldon sings "Do you really have to ask?"
After Sheldon fails to solicit the Rock N' Roll Hall of Fame for help, Penny tells him that he's got a song stuck in his head, which happens to everyone. But Sheldon isn't "everyone", since he has an eidetic memory and should be able to remember everything. Sheldon thinks something is wrong with him and that this might be the beginning of his dreaded "descent into madness".
Meanwhile, Amy calls her ex-(sort of) boyfriend Dave and asks him out another date. Dave eagerly agrees and even cracks a funny joke or two about his lack of appointments on his calendar
Over at Howard's lab, Raj tells Howard that he set up a Facebook page for their "geek rock" band, Footprints on the Moon. While Howard's upset at first (thinking that Raj is trying to take over the band), he quickly changes tune when Raj points out they have their first fan.
Back at the apartment, Sheldon tries playing the song on a keyboard to piece together the song stuck on his head. Penny suggests that Sheldon try taking his mind off the song, so Sheldon records a video to his "crazy future self", so he can still function after he's gone mad due to this darned unknown song.
Bernadette's a bit surprised that Amy's going on another date with Dave, but Amy assures Bernadette that Dave's obsession with Sheldon isn't any different than Sheldon's obsession with Sheldon. Amy says she'll be cooking dinner for Dave at her place, which Bernadette says is a big step. After all, a person's home is an intimate setting where "your underpants live". Amy says that this is a good thing since Sheldon turned her down, but Bernadette's not sure that Amy's first time having sex should be with someone that tall, since it's "like taking a driver's test in a bus." Bernadette's full of wisdom tonight.
Howard suggests to Raj that they respond to their band's sole fan and discovers their fan seems to be a cool dude, complete with tattoos, a job as a DJ and a hot girlfriend.
That night, Penny and Leonard listen as Sheldon continues to play the unknown song in his bedroom. They argue over who should go talk to him (mirroring the arguments parents have over who should tend to a screaming baby) and finally Penny gets out of bed to ask Sheldon to stop. Penny takes Sheldon's keyboard by force, but Sheldon gets the last laugh when he pulls out his tuba to keep plugging away at the song.
The next morning, Sheldon makes another video to his crazy future self, explaining the nuances of his living arrangement. It's an…interesting glimpse into Sheldon's head, since Sheldon views himself as a mischievous prankster whose fierce berating of anyone who sits in his spot as adorable.
Howard and Raj continues to Facebook-stalk their new fan. When they see he's checked into a nearby coffee shop, they decide to head there and meet him face to face. Bernadette points out that Facebook stalking is a little creepy, but that's nothing compared to Howard hunting down the guy's Twitter, Instagram, Snapchat and just about every other form of social media.
That evening, Dave and Amy enjoy a nice homecooked meal. Dave and Amy try to converse about anything that isn't Sheldon, but the "brilliant physicist" who Dave may or may not be obsessed over keeps finding ways to pop up in the conversation.
Meanwhile, the song continues to consume Sheldon's life. As Sheldon, Penny, and Leonard eat dinner, Sheldon continues to hum the song over and over, leading Sheldon to ask if they've ever dealt with anything so annoying in their lives. Sheldon resigns himself that, like all great scientists, he's destined to descend into madness. Sheldon points out all the famous scientists and artists who have gone crazy, including Pythagoras, Bobby Fisher and Brian Wilson from the Beach Boys. Suddenly, Sheldon has a flash of realization as he finally figures out where the song is coming from: it's a Beach Boys song called "Darlin".
Raj and Howard head to the coffee shop to check out their lone fan. The two briefly discuss whether they should approach the fan or let him discover them, but they're horrified when they see the dude picking his nose. Fantasy ruined, they rush out of the coffee shop
Sheldon tries to figure out why "Darlin" was stuck in his head, since he doesn't even like the Beach Boys. He realizes that the song's romantic lyrics reminded him of Amy and he rushes out of the apartment, presumably to interrupt Amy's date.
Back at Amy's apartment, things are slowly (very slowly) getting hot and heavy between Amy and Dave. The two share a single kiss, before Sheldon knocks on the door interrupting them. Sheldon gives a nonsensical speech about the song and how Amy's his heartworm, before Dave interrupts to explain to Amy that Sheldon wants her back. With Dave's encouragement, Sheldon tells Amy he loves her and the two share a passionate kiss while Dave watches and celebrates in the background. The episode ends with Dave letting himself out of Amy's apartment while Sheldon and Amy make out. After the door closes in his face, Dave remarks that he had a delightful evening.