Sheldon is beset with nostalgia when Leonard and Penny make a(nother) big decision when The Big Bang Theory 9×04 airs tonight.

According to CBS, Sheldon’s crisis in “The 2003 Approximation” is instigated when Penny and Leonard decide to official move in together.

“Sheldon learns of an impending change in his living arrangements and decides to revert back to 2003, a simpler time before he met Leonard and Penny,” the official synopsis reports. “Also, Howard and Raj form a band to play at the Comic Book Store.”

The new band — and their first instant-earworm single — are the focus of the preview for The Big Bang Theory 9×04.

On last week’s episode of The Big Bang Theory, Howard and Raj took Leonard and Sheldon to Mexico for a belated bachelor party. Or at least, they tried to — a flat tire ultimately led to the van exploding at a safe, but spectacular distance. Meanwhile, the girls forced Penny to tell her father about her wedding, which led to an upsetting revelation about her pet pig.

But it’s Sheldon who gets the bad news in the first clip from “The 2003 Approximation.”

In the second sneak peek, Howard and Raj get the genius inspiration for their new band — necessity.

Hey, for Stuart’s information, filk music is a thriving genre of creative and innovative songmakers. (Though to be fair, Raj and Howard probably won’t be among them.)

The still photos from tonight’s episode offer a glimpse at the guys’ attempt to channel their muses – and feature the return of Laura Spencer as Raj’s girlfriend Emily.

The Big Bang Theory 9×04, “The 2003 Approximation,” airs tonight at 8:00 p.m. on CBS.

What are your hypotheses for ‘The Big Bang Theory’ 9×04?