The geeks of "The Big Bang Theory" will return to CBS next month with its ninth season premiere, and it will answer one of the most difficult mysteries for the typically brilliant theoretical physicist Sheldon Cooper (Jim Parsons). 

Sheldon will allegedly mull over the decision of his girlfriend Amy Fowler (Mayim Bialik) to take a break from their relationship just before he was about to propose to her during the finale of season 8. Since relationship problems are not one of his fortes, Sheldon will try to understand why women would choose to spend time apart from their partners, and how long that time apart should last.

Meanwhile, Sheldon's roommate Leonard Hofstadter (Johnny Galecki) and Penny (Kaley Cuoco) are on their way to Las Vegas to elope after being taunted by Sheldon and Amy, but Leonard's confession during the season 8 finale titled "The Commitment Determination" — that he drunkenly kissed another woman two years ago while onboard a boat on the North Sea  — could possibly change Penny's mind. 

Penny might contemplate forgiving Leonard for his past mistake, especially since they were not yet engaged when that happened. But according to a previous report by TVLine, things will become more complicated for the engaged couple when the woman from the North Sea expedition suddenly appears this season. 

According to the report, "Mom" star Melissa Tang will portray the role of Mandy, the woman who shared a kiss with Leonard during the expedition. She will reportedly appear in the second episode of season 9, which could cause a further rift between Leonard and Penny. 

The official synopsis for the season 9 premiere episode of "The Big Bang Theory" reads:

Brilliant Sheldon is confronted by a mystery of the universe he cannot unravel: when a woman wants time apart to think, exactly how much time does that mean, and is there any way to hurry the process along? In Las Vegas, Penny and Leonard march closer to marching down the aisle, but has Penny gotten over Leonard's infidelity? And if so, will he do anything to un-get her over it?"

The episode will air on Monday, Sept. 21.