Big Bang Theory
“The Big Bang Theory” Season 9, episode 21 will see Sheldon (Jim Parsons) get into a heated argument with Leonard (Johnny Galecki) during a viewing party they’re hosting for the gang. CBS

In a season that’s been fraught with interactions between the characters that are trying to make it work in a romantic relationship together, “The Big Bang Theory” will return to its roots when it comes back on April 21 for episode 21 of Season 9. The hit CBS sitcom will get back to basics and re-examine the friendship between Sheldon (Jim Parsons) and Leonard (Johnny Galecki).

According to the show’s official plot synopsis from CBS, the episode, titled “The Viewing Party Combustion,” will live up to its name when a group get together, presumably for the premiere of “Game of Thrones” based on the below promotional photo, is marred by an argument between the two very old friends. Although things start out small, it eventually escalates as things often do when Sheldon is involved.

Big Bang Theory
“The Big Bang Theory” Season 9 will feature a viewing party in which Stuart (Kevin Sussman) shows up to Leonard’s (Johnny Galecki) place in costume during episode 21. CBS

Pretty soon the bout forces Penny (Kaley Cuoco), Howard (Simon Helberg), Raj (Kunal Nayyar), Amy (Mayim Bialik) and Bernadette (Melissa Rauch) to choose sides. The obvious choice in a battle between the two is Leonard, who doesn’t spend a majority of his time making life difficult for the other members of the group in question. However, with no word yet on the issue that will divide the two, it’s anyone’s guess how the gang’s loyalties will fall.

It would make sense for things to still be a little tense between Leonard and Sheldon after the he threw a metaphorical hand grenade into his relationship with Penny last week. During the episode, titled “The Big Bear Precipitation,” Leonard, Penny, Sheldon and Amy all took a romantic trip to the woods where a rainstorm forced them to stay inside and play drinking games.

During a rousing round of “Never Have I Ever,” Sheldon went for the throat and revealed to Penny that Leonard keeps a secret bank account because he believes his wife’s credit card debt is too insurmountable. Although the couple patched things up with Penny revealing that she needs a career change, it’s possible that Leonard is still carrying some resentment toward his loud-mouthed friend.

Fans who want to finally figure out what the rub is between the two characters will have to wait until “The Big Bang Theory” returns from its brief hiatus on April 21.