"The Big Bang Theory" is returning to CBS on Monday, 21 September 2015, with the episode "The Matrimonial Momentum" that will feature Leonard and Penny's wedding.

The episode can be watched online on www.cbs.com/shows/big_bang_theory/ or on www.cbs.com/all-access at 8 pm EDT.

Leonard and Penny's wedding won't be a smooth one, as Penny will be struggling to deal with Leonard's confession that he kissed another girl during a scientific expedition.

"It's the kind of thing that you wouldn't expect to happen and that can be a little jarring," showrunner Steve Molaro told The Hollywood Reporter. "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."

In the meantime, Amy and Sheldon will also be dealing with their relationship troubles. The previous season ended with Amy requesting a time-off from Sheldon to analyse where their relationship was going, and it looks like Sheldon isn't too pleased with the new arrangement.

According to an extended plot summary of "The Matrimonial Momentum" available on "The Big Bang Theory" wiki page, Sheldon will put pressure on Amy to get back with him. However, Sheldon's condescending behaviour and his remark about Amy's biological clock ticking away will force Amy to finally call it quits on their five-year-old relationship.

"He's having a lot of mixed feelings. He's mad at her, he wants her back, he's confused. There's a lot going on in his head and in any given scene, it can bounce back and forth," Molaro told TVGuide.

Interestingly, the showrunner doesn't think either Amy or Sheldon should be blamed for the current state of their relationship. While Amy wants more from the unorthodox romance, Sheldon feels they have come a long way in the past five years they've known each other.

The synopsis of "The Matrimonial Momentum" reads:

"Penny struggles with Leonard's confession that he kissed another woman as they prepare to get married in Las Vegas. Meanwhile, Sheldon doesn't know what to do after Amy pushes pause on their relationship."