After a bachelor party gone wrong, the gang will be up to something more fun and less crazy in this week's "The Big Bang Theory" season 9 installment.

Titled "The 2003 Approximation," Sheldon will have to part ways with his best bud Leonard, who is now moving out of their unit and into a new one with his wife Penny.

For a sea of reasons, Sheldon is not up for the idea at all. For one, Sheldon is going through a really tough time with his relationship with Amy at a standstill. Without someone to hang out with, his thoughts could quickly drift to the love of his life.

In addition, there are "The Big Bang Theory" season 9 spoilers hinting that Amy will have a new love interest, one who's good-looking and intimidating. If this is bound to happen, Sheldon will not have the best time for sure, especially with Leonard no longer around.

Also, Leonard has been his roommate in what feels like forever. But now that his best friend is leaving him, Sheldon will find it very difficult to adjust. He is not the only one unhappy about it, though. The newlyweds seem to be sad about leaving him too.

Nonetheless, the couple is going through the process. As expected, Sheldon will not be able to deal with this easily. In this "The Big Bang Theory" season 9 episode, he decides to revert to the time before he crossed paths with Leonard and Penny, the time when everything was "simpler."

As he does this, he will also make an attempt to sway Amy into becoming his new roommate, and while many fans think this a great idea, Sheldon's ex-girlfriend is not the least bit happy about this offer.

Meanwhile, this week's "The Big Bang Theory" season 9 episode will also see Raj and Howard form a band with Stuart (guest star Kevin Sussman) to be able to perform at a comic book store. Their first single sounds promising – "Thor vs. Indiana Jones."

"The Big Bang Theory" season 9 episode 4 "The 2003 Approximation" airs Oct. 12.