"The Big Bang Theory" season 8 ended with Leonard (Johnny Galecki) and Penny (Kaley Cuoco-Sweeting) heading to Las Vegas to get married when Leonard killed the mood by admitting he had an affair during one of his previous expeditions. A report from TVLine confirms that the mysterious woman will be played by Melissa Tang. 

Melissa's character, Mandy, will shake things up in the second episode of season 9 but not in the premiere, indicating that she may first appear in a flashback focusing on her and Leonard's affair before being brought in to interact with the characters in the present day. 

Leonard's affair was never going to be brought up until Penny decided that since they were going to get married, they shouldn't have any more secrets between them. This compelled Leonard to admit his affair with Mandy during their North Sea expedition which took place several years back. 

The season is also described to take place just hours after the finale of season 8. This is unlike previous seasons which usually integrated the few months in between seasons into their story. This time jump might still occur in the premiere episode but only after showing the events that followed the season 8 finale. 

Tang is no stranger to shows produced by Chuck Lorre as she has a recurring role in his other show, "Mom," where she plays Suzanne while also appearing in an episode of "How I Met Your Mother" where she played the character Amanda. 

The premiere episode of season 9 is expected to follow Leonard and Penny as they either push through with their wedding or, if Leonard somehow manages to ruin things even further, cancel it; as well as the cliffhanger surrounding Sheldon (Jim Parsons) and Amy's (Mayim Bialik) breakup, which occurred before the former could propose. 

"The Big Bang Theory" season 9 premieres on Sept. 21.