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After driving to Vegas to get married, Penny (KALEY CUOCO-SWEETING) struggles with Leonard's confession that he kissed another girl on the North Sea in the ninth season premiere.

Leonard confronts the woman he kissed on the North Sea in an attempt to set Penny's mind at ease. And Sheldon tries to retaliate after his breakup with Amy, using "Fun with Flags."

The guys surprise Leonard by taking him to Mexico for a belated bachelor party weekend, while the girls force Penny to finally tell her family that she eloped with Leonard.

When Sheldon (JIM PARSONS) learns of an impending change in his living arrangements, he decides to revert back to 2003 - a simpler time before he met Leonard or Penny.

When the guys take fencing lessons from Barry Kripke (JOHN ROSS BOWIE), Sheldon finds himself in an awkward position when Kripke expresses interest in Amy.

In the midst of a nationwide helium shortage, Leonard and Sheldon are forced to make a black market purchase from a dealer (MICHAEL RAPAPORT) in order to complete an experiment.

Leonard and Penny are shocked to learn that Sheldon had an engagement ring for Amy. Meanwhile Howard and Bernadette disagree over remodeling his childhood home.

With Amy out on a date with a mystery man, Sheldon enlists the help of Howard and Raj to find a new girlfriend by posting online.

While Howard reluctantly agrees to volunteer at the soup kitchen with Bernadette, Raj and Emily, Sheldon and Amy spend Thanksgiving together at the Aquarium as friends.

After driving Leonard and Penny crazy by his desperate attempts to remember the name of a song stuck in his head, Sheldon has a revelation when he realizes why.

After more than five years of dating, Sheldon and Amy spend their first night together; BOB NEWHART returns as Sheldon's Obi-Wan to advise him.

Howard and Bernadette experience "Empty Nest Syndrome" after Stuart moves out, while Penny gets more than she bargained for when Leonard meets with a psychiatrist on her behalf.

After having to deal with Sheldon being sick, Leonard, Penny and the gang try to treat themselves to a Sheldon free weekend.

Sheldon's thrilled when his Meemaw (guest star JUNE SQUIBB) comes to visit, but his excitement quickly fades when she butts heads with Amy.

Sheldon and Amy host a special, live episode of "Fun with Flags" for Valentine's Day; while Raj is torn between Emily and the new girl he met at the comic book store.

While the gang celebrates Bernadette's pregnancy announcement, Howard freaks out, convinced that he'll need to make more money to provide for the baby.

After nine years together, the gang finally celebrates Sheldon's birthday with a few special guests - including Batman's ADAM WEST - on the 200th episode.

Howard has second thoughts about entering into a legal partnership with Leonard and Sheldon for a patent, while Raj gets dating advice from the girls after his breakup with Emily.

Amy is shocked at a revelation from Sheldon after she buys him a new laptop; Meanwhile, Leonard and Howard lie to their wives to attend an early screening of a movie.

Sheldon spills a secret of Leonard's on a weekend cabin trip they take with Penny and Amy; and Howard and Bernadette must draw the line when Raj goes overboard with pregnancy gifts

When a small argument between Leonard and Sheldon erupts into a heated fight, everyone must take sides during a group get-together.

Sheldon and Bernadette spend a night hanging out together when the rest of the gang go to a wine bar, where they run into Penny's old boyfriend, Zack (BRIAN THOMAS SMITH).

Penny struggles to bond with Leonard's mother when she comes for a visit. Meanwhile, Sheldon hires Stuart to spend the day with Amy while he's at a movie screening.

Chaos ensues in the season nine finale when Leonard's recently divorced parents (JUDD HIRSCH and CHRISTINE BARANSKI) and Sheldon's devoutly religious mother come to town.