In the upcoming "The Big Bang Theory" season 9 episode 3, things don't go as planned for the boys on their way to celebrate Leonard's (Johnny Galecki) bachelor party in Mexico, while Penny (Kaley Cuoco) feels the pressure to fess up about eloping with Leonard.

In the previous episode of "The Big Bang Theory" season 9 episode 2, entitled "The Separation Oscillation," Leonard's confession of his kissing another woman threatened their relationship. In a bid to make things right, Leonard decided to meet up with Mandy (Melissa Tang), the girl she met during his North Sea expedition trip, just to assure Penny that there was nothing to worry about. Meanwhile, Sheldon (Jim Parsons) filmed an episode of "Fun with Flags" alone after his break-up with Amy (Mayim Bialik), who got mad about the comments Sheldon made regarding their break-up on his show.

In the new episode synopsis of "The Big Bang Theory" season 9 episode 3, entitled "The Bachelor Party Corrosion," Sheldon, Howard (Simon Helberg), and Raj (Kunal Nayyar) head to Mexico with Leonard for the surprise bachelor party they put together. Unfortunately, not everything goes according to plan when they encounter a flat tire. Now, boys are forced to use their scientific knowledge to solve the very "complex" problem of fixing a flat tire.

While the boys are busy dealing with their problem, Bernadette (Melissa Rauch) and Amy force Penny to tell her family that she has eloped with Leonard. Penny worries, though, that her parents might not approve of her decision.

Meanwhile, Penny and Bernadette help Amy get over her heartbreak with a make-over, hoping to take her mind off her break-up with Sheldon. Penny and Bernadette take turns playing "fashion police" as they critique Amy's fashion.

Will the boys be able to solve their problem and make it to Mexico for Leonard's bachelor party? And will Penny finally get the courage to break the news of her engagement to her family?

"The Big Bang Theory" season 9 episode 3 will air on Monday, Oct. 5, at 8 p.m. EST on CBS.