The Big Bang Theory season 9
The Big Bang Theory season 9 premieres on 21 September on CBSCBS

Television's hit comedy The Big Bang Theory returns with its ninth season on Monday, 21 September as Leonard and Penny get hitched in Las Vegas. Meanwhile, Sheldon tries to understand the break-up conundrum with Amy Farah Fowler.

The premiere episode is titled, The Matrimonial Momentum, where actor Laurie Metcalf returns as Sheldon's mother, Mary. Click here to watch the episode live stream online on CBS Go or click here to watch the episode online.

Season 8 ended with Amy asking for some time off from Sheldon, and the brilliant scientist was last seen holding an engagement ring. Meanwhile, on the way to Vegas to elope with Penny, Leonard confesses that he kissed Mandy during his North Sea expedition two years ago.

Speaking of Sheldon and Amy's relationship, executive producer Steve Molaro told TV Guide, "I don't think it's a spoiler to say they don't make up by the end [of the episode]. It's rough on both of them."

"He is a man of science and he is being presented a problem of the heart that is not easily solved. Problems that are not easily solved by logic can be extremely frustrating for him. That's something new that he's gonna be dealing with," Molaro revealed.

"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."

The producer also teased that despite Leonard and Penny going through with their wedding, their honeymoon phase is over quickly when Penny brings up Mandy on their wedding night. "Leonard tries to diffuse the situation, but it's clearly not working. It's not the way you want your wedding day to go," Molaro said.

The infidelity issue will be addressed in episode 2, when Melissa Tang guest stars as Mandy. "He's doing all he can to clear the air. We don't shy away from it at all the first few episodes," added Molaro. Here is the official synopsis for episode 1:

After driving to Vegas to get married, Penny struggles with Leonard's confession that he kissed another girl. Also, Sheldon doesn't know how to act after Amy pushes pause on their relationship, on the ninth season premiere of The Big Bang Theory.