The season 9 premiere of "The Big Bang Theory" on CBS will see Leonard and Penny finally saying "I Do" despite the fact that Penny was miffed at her fiancée kissing someone else during his North Sea expedition a couple of years ago.

Although Penny said she was cool with Leonard's little mistake, Leonard will soon realise after their wedding that kissing Mandy has serious repercussions. Their wedding night will be a challenging one, as Leonard and Penny will soon have a serious talk about Mandy and Leonard's relationship.

"I think when you're Penny, and you have an image that you've cultivated of a person for eight years now and those two things aren't aligning anymore — that can be a difficult thing to deal with," showrunner Steve Molaro told The Hollywood Reporter.

While the Leonard-Penny wedding will please a lot of fans, Sheldon is someone who probably won't be as enthusiastic. Sheldon is opposed to any kind of change, and the previous season saw him packing his bags and leaving when Sheldon and Penny announced they were engaged.

It remains to be seen how he will react to the wedding, especially now that he is in a vulnerable state as Amy dumped him. But one thing that will definitely be addressed in the first few episodes is their living situation.

Will Sheldon allow Penny and Leonard to move in together? "It's tricky, sure — Leonard and Penny need to move forward, but at the same time we have a broken-hearted Sheldon (Jim Parsons), who is sort of their little brother and child and it's tough to ignore that for them," Molaro pointed out.

He also added that there are three people in this marriage – Leonard, Penny and Sheldon.

"They're trying to all figure it out. We jokingly alluded to the fact that in some way the three of them are kind of married and I think the three of them will need to figure out how this is going to work."

"The Big Bang Theory" returns for season 9 on Monday, 21 September at 8 pm EDT.