Leonard tries to ease Penny's mind; Sheldon plans a special episode of "Fun With Flags"; Bernadette feels bad about keeping a secret.
A flat tire during Leonard's bachelor party weekend in Mexico tests the guys' skills; Penny is compelled to tell her family that she and Leonard eloped.
An impending change to his living arrangements inspires Sheldon to revert to 2003, before he met Penny and Leonard; Raj and Howard form a band.
Barry Kripke expresses interest in Amy while leading a fencing lesson for the guys; when Stuart tries to attract female customers to the store, Penny, Amy and Bernadette help out.
Leonard and Sheldon go to great lengths to secure the helium they need during a nation-wide shortage; Bernadette and Penny try to find Amy a new man on a dating app.
Sheldon struggles to keep his emotions in check when he is interviewed for a documentary about Spock; Penny and Leonard make a surprising discovery; Wolowitz and Bernadette get into a disagreement about remodeling his childhood home.
Sheldon, Wolowitz and Koothrappali post an ad online to find a new girlfriend for Sheldon; Penny and Bernadette decide to spy when Amy is secretive about who she is dating.
Leonard and Amy make an unexpected trip to Arizona to fetch Sheldon; Penny interviews for a job at Bernadette's company; Stuart's relationship with Mrs. Wolowitz disturbs Howard.
When Sheldon must teach a class, Howard decides to take it; Amy takes advantage of the tension between Penny and Bernadette.
Wolowitz feels nervous when he is asked to throw the first pitch at a Los Angeles Angels game; Penny and Leonard want to prove Sheldon wrong after he says he and Amy are the better couple.
Raj's honesty comes back to haunt him when his new girlfriend is unfriendly to Penny; the guys think about becoming part owners in Stuart's store.
During their weekend in Las Vegas, Bernadette and Amy accuse Penny of being a buzz kill; the guys try to invent the next big thing.
Raj and Sheldon try to simulate a dark-matter research expedition; after a fight about money, Penny and Leonard turn to Bernadette and Wolowitz for advice.
The guys try to help when Penny is faced with an awkward dilemma at work; Bernadette and Amy discuss sexuality and female scientists.
Sheldon feels pressured to participate in romantic traditions when the gang re-creates a high-school prom on the roof.
Leonard has minor surgery on his nose, causing Sheldon to overreact; Bernadette and Howard work on their marriage.
Sheldon says goodbye to Fun With Flags; Howard, Leonard and Raj look through their dead professor's research; Bernadette learns her co-workers' true feelings.
Leonard and Wolowitz clash over a work emergency; Amy throws a Victorian Christmas party which Sheldon resents.
When the guys agree to do something the girls like, Sheldon and Leonard end up having a confrontation.
Sheldon asks his friends to challenge him in hope of achieving a scientific breakthrough; Wolowitz invents a game about Raj.
Sheldon and Leonard must defend themselves from an online bully's criticism after they co-publish a paper; Bernadette, Penny and Amy confront embarrassing moments during a girls night.
While the gang waits for the reopening of Stuart's comic book store Wolowitz gets some shocking news; Penny has trouble taking her own advice; Leonard and Raj think they spot Nathan Fillion at a restaurant.
Penny and Sheldon design an experiment -- as a joke -- to make participants fall in love; Howard and Bernadette get into trouble at the airport.
Amy finds out that Sheldon applied for a one-way mission to colonize Mars; a trip to the pet store leads to a fight; Leonard has a surprise for Penny; Raj is busted while snooping in Emily's apartment.
A magazine article only credits Sheldon in an article about a paper he co-wrote with Leonard; a blackout at Mrs. Wolowitz's results in a final dinner in her honor.
When Sheldon and Leonard are asked to speak at UC Berkeley, they take a detour in hope of meeting an idol; Raj and Amy try to settle a dispute between Howard and Bernadette with a game of pingpong.
Sheldon and Amy build a blanket fort -- and come to a crossroads in their relationship; Wolowitz meets a half-brother he didn't know he had; Leonard and Penny have an argument during an appearance on Wil Wheaton's podcast.
Amy and Leonard try to become better acquainted; Bernadette, Howard and Mrs. Wolowitz spend the weekend together.
Sheldon takes acting lessons from Penny in hope of becoming a better teacher; Koothrappali fantasizes about his friend's girlfriend.
A wealthy donor makes Leonard question what he is willing to do for science.
Wolowitz considers taking things with Bernadette to the next level; Leonard and Raj's sister rekindle their relationship.
Sheldon is upset when he realizes Leonard is the center of their group of friends.
Leonard has to choose between Penny and his new girlfriend, Priya; Howard's magic trick drives Sheldon crazy.
The guys try to figure out who hacked Sheldon's online-game account.
Amy and Sheldon spread gossip as part of an experiment; Howard decides to take a big step in his relationship with Bernadette.
Priya dissects the roommate agreement; the ladies take Sheldon dancing.
Howard's mom winds up in the hospital after hearing news of her son's engagement to Bernadette.