'Big Bang Theory' Season 9 Spoilers: Show Casts Mystery Woman Leonard Kissed; Will It Ruin His Wedding Plans With Penny?

Leonard (Johnny Galecki) and Penny’s (Kaley Cuoco-Sweeting) honeymoon might be short-lived on "The Big Bang Theory," if they finally get to that point. A blast from Leonard’s past is expected to appear in episode 2 of Season 9.

TVLine reports that Melissa Tang has been cast as Mandy, the woman Leonard kissed while on a North Sea expedition. Penny was made aware of this just before the couple ran off to get married in Las Vegas at the end of last season.

Tang has also appeared in “Mom," "How I Met Your Mother," and "New Girl."

Leonard confessed to kissing a woman in the Season 8 finale because he wanted to go into their marriage with no secrets, but Penny was worried that he was trying to sabotage their wedding plans. Will the couple finally get married in Season 9 or will Penny not go forward with it because Leonard was unfaithful? Viewers will have to tune in to find out.

Season 9 is also going to be packed with drama for Sheldon (Jim Parsons) and Amy (Mayim Bialik). When viewers last saw Amy she requested that they take some time away from each other to reevaluate their relationship. It was later revealed in the Season 8 finale that Sheldon had a ring and was planning on asking Amy to marry him.

In July, executive producer and writer Steven Molaro revealed that fans will find out when Sheldon bought the ring in the Season 9 premiere. Fans will also get to learn about Amy's family and her past.

“The Big Bang Theory” Season 9 will kick off at 8 p.m. EDT Sept. 21 on CBS.