Howard dresses as Sheldon for Halloween at work, hurting Sheldon's feelings. When Howard does not apologize, Amy and Sheldon dress as Bernadette and Howard at Penny and Leonard's party, angering Bernadette. Sheldon and Bernadette later bond over their painful childhood memories. Leonard is upset that Penny does not seem to remember that their first kiss was on Halloween. Penny later admits she does remember, but does not like to think of their first kiss as an unromantic drunken hookup. She likes to imagine their first kiss was one they shared on Leonard's birthday, which pleases him.

Mayim Bialik as Amy Farrah Fowler (guest star season 3, starring seasons 4–12):[52] A woman selected by an online dating site as Sheldon's perfect mate.[53] Amy is from Glendale, CA. While she and Sheldon initially share social cluelessness, after befriending Penny and Bernadette she eventually becomes more interested in social and romantic interaction. Her relationship with Sheldon slowly progresses to the point at which Sheldon considers her his girlfriend, eventually they get married. Amy believes she and Penny are best friends, a sentiment that Penny does not initially share. Amy has a Ph.D. in neurobiology.
The Big Bang Theory signs off for good on Thursday, May 16, meaning that there's only one last bazinga left in the pipeline before we're forced to say farewell to Sheldon, Leonard, Penny, and the gang after 12 laugh-filled seasons. And while there's still a few questions left to be answered by the big one-hour series finale—Will the elevator ever be fixed? And will we ever learn Penny's maiden name?—there's a bigger one looming for when the credits roll and it's all over. 
I read through the article and all the comments (which makes me miss TBBT even more and it hasn’t been off the air a week yet) and noticed that 3 of my favs aren’t mentioned. (1) episode where a bully (Todd Sornecki) steals Sheldon’s online trophies and Penny kicks him in the nuts for Sheldon; (2) the episode where they reveals each others secrets (Leonard slept with Priya; Raj used Sheldon’s tooth brush; Howard told Raj foreigner give gifts to Americans for certain holidays and Sheldon put moths in Leonards food “for science”; (3) the episode where Amy and Sheldon go to the aquarium “as friends”, when Sheldon tells Amy “I can handle a lot of things Amy but getting over you isn’t one of them, I think we should just be friends” it breaks my heart everytime; (4) the episode where Priya buys Leonard glass and he walks into the cafeteria and causes mass chaos, that’s a classic; (5) the episode where Priya ruins the roommate agreement between Sheldon and Leonard, when Leonard buys Sheldon Greek food it’s hilarious. Sheldon gets back at Leonard & Priya by threatening to send an email to Priya’s parents telling that “she’s in a relationship with the whiter than marsmellow fluff Leonard Hofstedter”. Ok just one more, when they all take a road trip to speak at the science convention and Bernadette runs into her former professor/lover played by Rick Foxx and Sheldon banished Penny to the “love car” and Bernie and Howard sing Sonny & Cher “I got you Babe” is funnier than hell. I still crack up when I see Raj look down at Howard’s junk to measure Howard’s junk against Rick Foxx’s, I can’t believe the sensors let that through.
Leonard's mother Beverly visits, and he is thrilled when she takes an interest in his work. However, when he learns that she is doing research on him for a new parenting book, he is furious. He prepares to tell her how she let him down over the years, but instead chooses to accept and forgive her, and forgive himself for holding onto that resentment for so long. Beverley is touched that he forgave her and they share a genuine hug. Meanwhile, Anu gets a job offer in London. Raj prepares to fly out and propose to her, ready to follow her if she accepts. Howard stops Raj at the airport, saying there is a better match out there for him, convincing Raj to return home. Stuart and Denise are bothered by her creepy roommate Mitch, while Bernadette and Howard are tired of Denise constantly staying overnight with Stuart at their house. Stuart says he is ready to move in with Denise, and they say they love each other while convincing Mitch to move out.
Howard and Raj are still fighting, so Howard dis-invites Raj to Halley's birthday party. However, when Stuart pulls out as party planner at the last minute, Raj agrees to plan the party, which doesn't turn out too well due to Halley being asleep, Bernadette being on bed rest, and no people turning up. When Howard and Raj get into a shoving match in the party's bounce house, they end up letting out their frustrations, but also having fun and reconciling. Leonard is depressed upon receiving his successful brother's Christmas card and attempts to make a list of what he and Penny accomplished this year, not coming up with much. This prompts them to think about their plans for the future, so they decide to go on trips and achieve more out of life together before starting a family. Sheldon cooks Amy a Little House on the Prairie-themed birthday dinner, which gives them both food poisoning. They eventually recover enough to attend Halley's birthday party with the others, where they have fun in the bouncy castle and then have sex in Howard and Bernadette's house.
Sheldon's mother Mary refuses to attend his wedding unless he invites his older brother George, with whom he is no longer speaking. Sheldon and Leonard fly to Texas to talk to George, who now owns a chain of tire stores. He refuses to attend and later explains to Leonard and Sheldon that the family sacrificed a lot to fund Sheldon's education, and he was stuck caring for their mother and sister Missy after their father died while Sheldon was in college. Sheldon never thanked him for this. Leonard takes George back to the hotel room where Sheldon apologizes for what he did and the brothers reconcile. George agrees to attend the wedding. Meanwhile, Howard and Bernadette's children contract pinkeye, infecting everyone except Penny, much to Amy's fury.
Johnny Galecki as Leonard Hofstadter:[42] An experimental physicist with an IQ of 173, who received his PhD when he was 24 years old. Leonard is a nerd who loves video games, comic books, and Dungeons & Dragons. Leonard is the straight man of the series, in which he shares an apartment in Pasadena, CA, with Sheldon. Leonard is smitten with his new neighbor Penny when they first meet, and they eventually marry.

Cosmologist Stephen Hawking made a short guest appearance in the fifth-season episode;[57] in the eighth season, Hawking video conferences with Sheldon and Leonard, and makes another appearance in the 200th episode. In the fifth and sixth seasons, NASA astronaut Michael J. Massimino played himself multiple times in the role of Howard's fellow astronaut. Bill Nye appeared in the seventh and twelfth seasons.

"The Plagiarism Schism" -- Pictured: Howard Wolowitz (Simon Helberg) and Bernadette (Melissa Rauch). Kripke (John Ross Bowie) has proof that Dr. Pemberton (Sean Astin) plagiarized his thesis in college, and Sheldon and Amy aren\'t sure if they should turn him in or not. Also, Wolowitz is happily surprised to learn that Bernadette wasn\'t the only waitress at the Cheesecake Factory who had a crush on him back in the day, on THE BIG BANG THEORY, Thursday, May 2 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Photo: Michael Yarish/Warner Bros. Entertainment Inc. © 2019 WBEI. All rights reserved. (Photo: Michael Yarish, Warner Bros.)

After Sheldon reminds the guys about the time they mined Bitcoin without him, the guys go on a quest to find their suddenly valuable missing cryptocurrency. They realize the currency is on a laptop which Leonard gave to Penny when they dating, which she then gave to her ex-boyfriend Zack. After Leonard and Penny go to Zack's apartment, he shows them a seven-year-old video that Penny recorded on the laptop soon after she broke up with Leonard. Leonard is touched by the video which shows a drunk Penny lamenting the break-up of her relationship with him.
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