In "The Separation Oscillation", the second episode of season 9 of "The Big Bang Theory", Leonard will attempt to make things right with Penny by contacting the girl he kissed years ago while on a boat trip.

What brought about Leonard's change of heart is a nightmare involving Sheldon and Penny. According to plot spoilers, Leonard will have a dream about Penny and Sheldon making out, and this displeases him. Once he wakes up, he also realises how guilty he feels for kissing Mandy during the boating trip.

Actor Johnny Galecki, who plays Leonard, told The Hollywood Reporter that his character will put his head on the chopping block for this relationship because he genuinely feels guilty for hurting Penny.

"There are moments when I want to defend Leonard," Galecki said. "They were not together at the time and it was a kiss. It was influenced chemicals! (Laughs.) It can certainly be worse! At the same time, that Penny is hurt over the whole thing, it's hard to not judge him harshly. As far as Mandy, Leonard is doing his best to rectify what he's tarnished and is going to put his head on the chopping block for Penny for the relationship."

As for Sheldon and Amy's relationship, the former will attempt to make his ex-girlfriend jealous by lying about being with a number of women since their breakup.

Arriving at Amy's apartment with a box of her personal belongings to return, Sheldon tries to remind Amy they are a good team. However, she is still mad at him for his condescending remarks, and Sheldon fails to make her jealous or convince her to get back with him.

The synopsis of "The Separation Oscillation" reads:

Leonard confronts the woman who kissed in the North Sea in an attempt to reassure Penny. Meanwhile, Sheldon recorded a special episode of "Fun with Flags" (Fun with Flags) after his breakup with Amy. And Bernadette feels guilty about keeping a secret of Penny.

Season 9, episode 2 will be aired on Monday, 28, September, 2015 on CBS.