Plot

Bernadette and Wolowitz leave their kids for the first time; Penny and Leonard try to keep a secret; Sheldon and Amy stick together; and Koothrappali makes a new friend, as the gang travels together into an uncharted future.

Sheldon, Amy, Leonard, Penny, Howard, Bernadette, and Raj decide to go away together to attend a momentous event. Sheldon's self-centeredness proves the last straw for many in the group, stirring feelings for returning home. Meanwhile, Leonard and Penny struggle to keep the lid on some big news. For Howard and Bernadette, this trip will mark their first time away from the kids for an extended period of time, leaving Stuart and Denise acting as their sitters. It may be more difficult than they imagined to be away from the kids, especially due to the distance. And Raj, in the absence of Cinnamon (who is staying with Bert), meets a new friend.—Huggo

Sheldon and Amy finally win the Nobel; meanwhile Leonard and Penny keep mum about their big secret while Howard and Bernadette are hell worried of their children who are under the babysitting of Stuart and Danise. Raj finds a new friend on his flight to the Nobel Prize Ceremony.

Two months after receiving Sheldon and Amy's Nobel Prize news, Sheldon and Leonard are seen repairing the DNA model after 179.5 hours of work. Sheldon constructed the model as a genetic code for an advanced race of human beings. Penny and Amy come back from the tailor's with the dresses they're going to wear to the Nobel Prize ceremony in Stockholm, Sweden. Amy's dress had to be let in and Penny's had to be let out, which delighted Amy. Penny thanked her friends for the clothes and the airline tickets. The Coopers go back to Apartment 4B so Amy can show him the dress and Sheldon can practice his speech. As soon as Sheldon and Amy leave, Leonard asks Penny if Amy suspected anything on why she needed her dress let out. No. She is expecting. Also, she is not yet ready to tell her friends. It happened after she went drinking with Sheldon two months ago. She wanted to have sex and Leonard agreed to it.

Bernadette is talking to her son Neil about going away for a few days as she picks him up. Howard brings in Halley, who is a big girl and just went on the potty. They feel good about leaving them with Stuart and Denise.

Raj is leaving Cinnamon in the care of Bert. He hopes that he can attract girls with her. Bert also states he'll get another one just like her, in case she dies on him. Just kidding.

Amy is in her dress and tops it off with her tiara. Sheldon begins reading his speech very fast to get it under 1.5 hours. Amy suggests cutting the story of his childhood, even though he put it in to relate to everyone. Amy feels that the Nobel Prize should be accepted with humility; however, Sheldon thinks that humility is for all those other Nobel Prizes categories peace, chemistry, literature and not the one for physics, which displeases Amy.

Before heading to the airport at their pre-trip meeting, Sheldon explains that they are all ambassadors for him and Amy and that they must be on their best behavior. Amy gives them tips on how to interact with the Swedish people. Be punctual. Shake everyone's hand when social events start and are completed. They get ready to leave and try to find various ways to get people and luggage into the newly operating elevator. Finally, through Leonard's suggestion, the luggage goes down the elevator by itself and the gang ends up walking down the stairs for the last time.

On the plane, Bernadette is doing fine watching Jack Reacher movies, until Howard mentions how they are getting further and further away from their children. Raj suspects that Buffy the Vampire Slayer (Sarah Michelle Gellar) is sitting next to him, even though Howard denies it. Penny is feeling sick and runs back to the lavatory. Sheldon is worried that she has a stomach bug, but Amy figures that it's just air sick. Penny comes back out and then runs back in. Sheldon goes to Leonard to complain about Penny and that he doesn't want to get sick on the greatest day of his life. Leonard assures Sheldon that he won't get what she has. Then Penny shows up and tells him that she is really pregnant as Leonard breaks into a big smile. Sheldon is happy that he won't get sick and shocks them by just leaving. He goes back and tells Amy that Penny is just pregnant, who is also flabbergasted.

A peeved Leonard goes back to Sheldon complaining that the woman who has loved and cared for him for twelve years says that she is pregnant and all he cares about is not getting sick. Meanwhile, Amy isn't happy that Penny didn't tell her best friend, namely herself. Bernadette stops by, finds out and calls that news fantastic while still being furious about being kept in the dark about it. Back with Sheldon, Leonard thinks that congratulations are in order. Sheldon reminds him that Penny didn't want any kids. Leonard explains that it happened anyway, and that they didn't want to upstage their day in Stockholm. Sheldon reacts that this is the Nobel Prize and any idiot can have a baby, using Howard as an example. Angered, Leonard calls him a selfish jerk and to hell with him and his Nobel.

At their hotel, Bernadette Skypes with Stuart, who sounds like he lost Halley while playing hide and seek. It turned out that she also lost a tooth when she rolled down the stairs while giving Neil, who has a fever, his teddy bear. The Wolowitzes look worried.

In Penny's hotel room, she is eating everything, including pickled herring. Leonard is complaining that after all these years, Sheldon would for once consider other people's feelings. Someone is at the door and Penny hopes that it's her salted cod. Instead, it's Sheldon, who apologizes and congratulates them on the baby news. Howard joins them and says that they are thinking of heading home since he and Bernie are having a meltdown about their kids. Sheldon selfishly tells him to keep it together for his big day, which leads to Howard making up his mind and choosing to leave. Leonard and Penny are also heading home because of Sheldon's behavior if Penny can get pickled herring on the plane.

All dressed for the ceremony, Sheldon thinks that their friends are being incredibly selfish. Amy sarcastically tells him that he would be the authority on that subject, a rare instance of Sheldon detecting sarcasm insincere tone at the very least, Amy says the Nobel is never more important than their friends, who Sheldon says is abandoning them. Amy angrily yells they're all leaving because he broke their hearts even if Sheldon didn't mean it. The only reason they tolerate him is because they are friends and they know he isn't aware of what he is doing.

Penny is still sick from all the food she ate. She is in no shape to head home and they both want to be there for Sheldon and Amy. Penny says that no matter how thoughtless and selfish he is, she still loves Sheldon. Leonard points out that he was like their practice kid. Penny likens him to the first pancake that turns out wonky. Howard called and said that they are staying too. Her parents came over and took over for Stuart and Denise, with Leonard saying that Mike accidentally broke one of Stuart's ribs with a hug.

At the Nobel Prize ceremony, both Sheldon and Amy are presented their medals on stage, with the whole gang watching. The guys cheer and Raj is seen having escorted Sarah Michelle Gellar as his guest (not as his date, even though he was holding her hand, which he sheepishly releases after she points it out). In her acceptance speech, Amy says what an honor it is to get the Nobel Prize and tells young girls everywhere who are interested in science to go for it, as it is the greatest job in the world.

Sheldon steps up and mentions his very long self-centered speech, but then puts it aside, causing the gang to cheer. After momentarily looking surprised, Sheldon continues and says what an honor it is and that he wouldn't be there without the most important people in his life: his family and his friends. He was under the misconception that his accomplishments were his alone, but he came to realize that he had been encouraged, sustained, inspired, and tolerated by both his wife and the greatest group of friends anyone ever had. He has them stand up as he introduces them and even acknowledges Howard as an astronaut (finally giving him the respect that Sheldon rarely gave him). Sheldon then calls Leonard and Penny his dearest friends in the world. Recalling when they first met, he tells the audience that Leonard said the two of them would have smart and beautiful children, and since they are now expecting, he had no doubt that they would. Choking up, Penny thanks him, even though her parents didn't know yet and he had just inadvertently let the cat out of the bag. Sheldon thanks everyone and gives Amy a kiss as the audience applauds.

The last scene has everyone back at the apartment after the ceremony, eating take-out food as always and enjoying each other's company. Sheldon and Amy are still wearing their Nobel medals, while Leonard and Penny kiss as Leonard places his hand on Penny's stomach. An acoustic version of the theme song slowly plays as the screen fades to black, a gentle ending to a saga that "all started with a big bang."

Title reference: Sheldon thinking Penny is ill on the flight to Stockholm; a reference to Stockholm syndrome.