Depois de guiar até Vegas para se casar, Penny debate-se com a confissão de Leonard de ter beijado outra rapariga no Mar do Norte, na estreia da nona temporada.

Leonard confronta a mulher que beijou no Mar do Norte, numa tentativa de deixar Penny descansada. E Sheldon tenta retaliar, depois da rutura com Amy, usando 'Diversão com Bandeiras.

Os rapazes surpreendem Leonard, levando-o ao México para um fim de semana de despedida de solteiro atrasado, enquanto as raparigas forçam Penny a dizer por fim à família que fugiu para se casar com Leonard.

Quando Sheldon descobre uma iminente alteração às condições de vida em casa, decide voltar a 2003 - uma época mais simples antes de conhecer Leonard ou Penny.

Quando os rapazes têm aulas de esgrima com Barry Kripke, Sheldon sente-se numa posição incómoda, quando Kripke expressa interesse por Amy.

No meio de uma escassez nacional de hélio, Leonard e Sheldon veem-se obrigados a fazer uma compra no mercado negro, a um traficante, para completarem uma experiência.

Leonard e Penny ficam chocados ao saber que Sheldon tinha um anel de noivado para Amy. Entretanto, Howard e Bernadette discordam sobre a remodelação da casa da infância dele.

Com Amy fora, num encontro com um homem misterioso, Sheldon pede ajuda a Howard e a Raj para arranjar nova namorada online.

Enquanto Howard concorda, relutante, em ser voluntário na Sopa dos Pobres com Bernadette, Raj e Emily, Sheldon e Amy passam o Dia de Ação de Graças juntos, no Aquário, como amigos.

Depois de endoidecer Leonard e Penny com as desesperadas tentativas de recordar o título de uma canção que tinha na cabeça, Sheldon tem uma revelação, quando percebe o porquê.

Depois de mais de cinco anos de namoro, Sheldon e Amy passam a primeira noite juntos; Bob Newhart regressa como o Obi-Wan de Sheldon, para o aconselhar.

Howard e Bernadette sentem o 'Síndrome do Ninho Vazio' depois de Stuart se mudar de casa, enquanto Penny arranja mais lenha para se queimar quando Leonard vai ter com uma psiquiatra em nome dela.

Depois de terem de lidar com Sheldon doente, Leonard, Penny e o grupo tentam regalar-se com um fim de semana sem Sheldon.

Sheldon fica exultante, com a visita da avó, mas o seu entusiasmo rapidamente se desvanece, quando ela entra em conflito com Amy.

Sheldon e Amy apresentam um episódio especial ao vivo de 'Diversão com Bandeiras' no Dia de S. Valentim; enquanto isso, Raj sente-se dividido entre Emily e a rapariga que conheceu na livraria de B.D..

Enquanto o grupo festeja a notícia da gravidez de Bernadette, Howard entra em pânico, convencido de que terá de ganhar mais dinheiro para sustentar o bebé.

Depois de nove anos juntos, o grupo festeja por fim os anos de Sheldon com alguns convidados especiais

Howard sente dúvidas sobre entrar numa parceria legal com Leonard e Sheldon para uma patente, enquanto Raj é aconselhado sobre encontros pelas raparigas, depois de se separar de Emily.

Amy is shocked at a revelation from Sheldon after she buys him a new laptop. Also, Koothrappali sells out Leonard and Wolowitz after they lie to their wives in order to attend an early screening of a movie.

Sheldon spills a secret of Leonard’s when the two take a weekend cabin trip with Penny and Amy. Also, when Koothrappali goes overboard with pregnancy gifts, Wolowitz and Bernadette must draw the line.

Everyone must choose sides when a small argument between Leonard and Sheldon erupts into a heated fight during a group get-together.

The gang goes to a wine bar and runs into Penny’s old boyfriend, Zack. Also, Bernadette gets stuck spending the evening with Sheldon when she can’t go wine tasting with the group, and things don’t go as planned when Koothrappali introduces his new girlfriend, Claire.

Sheldon hires Stuart to spend the day with Amy when he’d rather go to a movie screening. Also, Leonard’s mother, Beverly, comes to town and Penny struggles to make a connection with her.

Chaos ensues when Leonard’s recently divorced parents, Alfred and Beverly, and Sheldon’s devoutly religious mother, Mary, come to town. Also, Wolowitz and Koothrappali think the government is out to get them when they are contacted about their guidance system.

After Sheldon’s mother and Leonard's father share an evening together, everyone deals with an awkward morning the next day. Also, Penny’s family arrives for the wedding ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.

Colonel Williams, an Air Force representative from the Department of Material Command sits down to talk with Wolowitz.

Tensions rise when the boys struggle to complete their government project on time and Sheldon tries an energy drink to stay awake. Also, Penny and Amy go to a “party” at Bert the geologist’s house, and Koothrappali learns Bernadette’s true feelings about her pregnancy when they clean out the future baby room.

When Amy’s apartment floods, she proposes a “cohabitation experiment” with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.

Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.

Penny is surprised to learn she has fans of her performance in "Serial Ape-ist" when she attends Van Nuys Comic Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment, and Wolowitz makes an impulsive purchase when he starts stressing out about the baby on the way.

In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment and they host an "on location" episode of "Fun With Flags" in Penny’s apartment. Also, Leonard lets Penny decorate Sheldon’s old room after he learns that she has been secretly moving Leonard’s collectibles into storage.

After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Koothrappali is embarrassed to tell the gang what the new woman he’s dating does for a living.

When Bert, a Caltech geologist, wins the MacArthur Genius fellowship, Sheldon is overcome with jealousy. Also, Wolowitz finds an old remote control Stephen Hawking action figure he invented.

Sheldon and Leonard try to divvy up their shared belongings, but can’t agree on anything. Also, Koothrappali and Stuart fight to be the most helpful during Bernadette’s final weeks of pregnancy.

Howard and Bernadette welcome their new baby and Sheldon and Amy celebrate Amy’s birthday.

Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas.

When Penny feels that Leonard is taking her for granted, she decides to go on a spa weekend with Amy. Also, Wolowitz and Koothrappali try to fix a squeaky floorboard in the baby’s room.

Sheldon beta-tests an MIT invention after hearing it can help him read people’s emotions. Also, Leonard can’t hide his feelings when Penny announces her brother is coming to town, and Raj invites his ex-girlfriends over to give him dating advice.

Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley.

Sheldon and Amy have their first fight since moving in together, and Koothrappali is shocked at the realities of financial independence when he stops receiving support from his father.

With Koothrappali cut off from his father’s fortune, he appoints Sheldon to help him get out of debt. Also, Penny considers joining Leonard at Comic-Con.

When Raj moves out of his apartment, Leonard and Penny offer him Sheldon's old room.

Leonard, Penny and Raj adjust to their new living arrangement, and Sheldon takes an interest in Amy’s work.

Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Wolowitz as well as Amy on the same day. Also, Bernadette is nearing the end of her maternity leave and must return to work.

Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.

Leonard isn’t thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.

After Leonard, Sheldon and Wolowitz celebrate the completion of the top secret air force project, they are met with an unpleasant surprise. Also, Amy is offered a summer position as a visiting researcher.

The gang is concerned when Sheldon’s former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.