Sheldon struggles to understand what is going on with Amy; Penny must decide if she can get past Leonard's bad behavior.
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 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.
Amy and Sheldon spend Thanksgiving at the aquarium -- as friends; Wolowitz volunteers at the soup kitchen with Raj, Emily and Bernadette; Penny realizes that Leonard knows her better than she thought.
Sheldon has a song stuck in his head; despite their awkward date, Amy asks Dave over for dinner; Koothrappali and Wolowitz are obsessed with a fan of their band.
After dating for more than five years, Amy and Sheldon spend the night together for the first time; Leonard, Wolowitz, and Koothrappali must decide what to do with an extra movie ticket.
Leonard agrees to meet with a psychiatrist (Jane Kaczmarek) on Penny's behalf; Sheldon and Koothrappali work together on an astronomical discovery; Wolowitz and Bernadette struggle with their feelings when Stuart moves out.
After dealing with Sheldon while he was sick, Leonard, Penny and the gang attempt to treat themselves to a Sheldon-free weekend.
Sheldon's excitement about his Meemaw's visit fades when she butts heads with Amy; Raj questions his relationship with Emily after meeting someone new.
Amy and Sheldon host a Valentine's Day edition of Fun With Flags; Penny and Leonard wrestle with getting older; Howard and Bernadette make a discovery in their hot tub; Raj is torn between Emily and another woman.
The gang celebrates news of Bernadette's pregnancy by singing karaoke; Wolowitz's reaction to the pregnancy announcement worries Bernadette; Wolowitz believes he needs to make more money to support his growing family.
For the first time in nine years, the gang celebrates Sheldon's birthday -- surprising him with a special guest.
Leonard, Sheldon and Wolowitz file for a patent on their new invention; Penny, Amy and Bernadette give dating advice to Koothrappali after his breakup with Emily.
Sheldon makes a surprise revelation when Amy buys him a new laptop; Leonard and Wolowitz get sold out by Koothrappali after lying to their wives to attend a movie screening.
Sheldon shares a secret about Leonard when the two go on a weekend trip with Penny and Amy; Koothrappali goes all out with pregnancy gifts.
During a group get-together, a minor argument between Sheldon and Leonard turns into a heated debate, leaving the rest of the gang to choose sides.
The gang runs into Penny's old boyfriend while out at a wine bar; Bernadette spends an evening with Sheldon; Koothrappali's introduction of his new girlfriend doesn't go as planned.
Sheldon wants to go to a movie screening, so he pays Stuart to spend the day with Amy; Penny has a difficult time connecting with Leonard's visiting mother.