Leonard and Sheldon are brilliant physicists, socially inept outside of the lab. It's up to free-spirited neighbor Penny to teach them a thing or two about life. While Leonard spends his days looking for love, Sheldon is quite content with their ever-expanding universe of friends, including fellow scientists Raj, Howard and Howard's new fiancee, Bernadette and Penny's "bestie" Amy Farrah Fowler.
The fifth season premiere picks up where the fourth season finale left off as the gang deals with the aftermath of Penny and Raj's night together. While Penny worries that she's screwed things up permanently with her friends, Sheldon takes command of the paintball team. Emmy (R) nominee CHRISTINE BARANSKI (The Good Wife) reprises her role as Leonard's emotionally distant mother.
A battle of wills between Sheldon and Penny leaves Amy caught in the middle, while Leonard tries to spice up his long-distance relationship with Priya (recurring guest star AARTI MANN), who has moved back to India.
Leonard attends a co-worker's wedding as Amy's date. Meanwhile, Howard tries to convince Bernadette to live with his mom after they get married by suggesting a trial run with the two of them staying with his mom over the weekend.
Penny sets Raj up with a deaf girlfriend of hers, solving his problem of talking in front of a woman, and Raj falls for her instantly. He showers her with lavish gifts until his parents soon cut him off and Raj discovers love is not enough for her when she dumps him.
Sheldon's old nemesis, WIL WHEATON (guest starring as Himself), returns and invites everyone to a housewarming party. Sheldon threatens to "defriend" Leonard if he attends. But when he hears that BRENT SPINER (guest starring as Himself), the actor who played Commander Data, will be there, Sheldon decides to go to the party after all...only to discover new levels of treachery. Meanwhile, Howard gets an out-of-this-world opportunity that Bernadette reacts to strongly.
Sheldon's mother, Mary, comes to visit and the entire gang loves spending time with her, much to Sheldon's chagrin. He refuses to believe it's jealousy but, when he comes down with the common cold like a "common man", he's gleeful that mommy is there to take care of him and sing him "Soft Kitty."
Leonard meets a really cool girl, Alice, in the comic book store who immediately expresses an interest in him. The more time he spends with her, the more conflicted he feels about his relationship with Priya. In the end, he confesses to Priya that all he and Alice did was kiss and it will never happen again but Priya tells him she also messed up by sleeping with her ex-boyfriend. They both agree that perhaps the long-distance thing isn't working out so well. In honor of Halloween, the guys pull pranks on Sheldon throughout the episde. He tries to retaliate but they always seem one step ahead.
Amy is heartbroken when she learns Penny and Bernadette went to look at bridesmaids dresses without her. Sheldon is having trouble getting in contact with Amy, so Leonard drives him over to check on her. They find Amy sadly playing "Everybody Hurts” on the harp. Amy explains that she’s sad because Penny and Bernadette went shopping for bridesmaid’s dresses without her. Sheldon asks her what he can do to help and she says, "comfort me.” She then tells him she craves human intimacy and physical contact. When Sheldon reminds her that theirs is a "relationship of the mind,” Amy suggests that maybe one night of passion might blot the pain. Sheldon instead offers to stroke her head for five minutes or until she falls asleep – whichever happens first. Wolowitz tells Penny and Bernadette what happened with Amy and they tell him that Amy has just been taking her bridesmaid duties to an extreme but go to apologize -- Amy doesn't accept. Later, Amy calls Sheldon drunk and Sheldon blames Leonard for Amy's behavior since he brought Penny into the group. In the end, Bernadette tells Amy she wants her to be her made of honor and Amy enthusiantically accepts, taking out her video camera to capture the moment.
Leonard and Penny try to hang out alone as friends for the first time since they broke up. Sheldon's phobias are put to the test when a bird camps out on the living room window sill.
When Stuart asks Amy out on a date, Sheldon finds himself jealous. Everyone is shocked when Sheldon officially asks her to be his girlfriend.
When Leonard is contacted by a former high school bully through Facebook, he cringes at the memories. The former classmate is going through some rough times and reaches out to Leonard, not realizing that he had made Leonard’s life hell. Leonard agrees to meet with his former bully and the guys decide to accompany him for support. When Leonard confronts him about how mean he was, the guy feels badly and goes into a depression, making Leonard feel guilty. In the meantime, Amy and Bernadette help Penny choose a philanthropic endeavor.
Amy gets professional recognition and Sheldon, in his self-centered way, couldn't care less. Penny, Koothrappali and Leonard go about trying to make Sheldon a better boyfriend by getting him to realize that sometimes in a relationship you need to pretend to care - even when you really don't. At the same time, Wolowitz and Bernadette realize they need to discuss their feelings about children, after Howard performs a magic show at his nephew's birthday party with Bernie as his assistant. We learn Bernadette can't stand being around kids.
Leonard decides to ask Penny out on a date, and they both agree to give their relationship another chance. They argue over dinner and remember why they didn't work in the first place, but they end up sleeping together afterwards anyway. They agree to secretly continue seeing each other under the condition that they act like friends around everyone else, just so that they can make sure the relationship works. Eventually it is revealed that this is just a fantasy for Leonard, but it does inspire him to actually ask Penny out. She agrees to go to dinner with him, and their relationship begins anew.
Leonard and Penny decide to put their relationship through a 'beta test,' which allows them to report the problems they have with each other as 'bugs,' like a programmer would do with computer software. When they exchange lengthy lists of relationship bugs, Leonard takes it to heart and decides to take Penny to on a real date to a gun range, where he promptly shoots himself in the foot. Meanwhile, Koothrappali buys an iPhone 4s from the mall and falls in love with Siri. Wolowitz and Bernadette become concerned when he starts treating his phone like a real woman. Sheldon and Amy try to achieve internet fame by creating "Sheldon Cooper Presents: Fun with Flags," a video podcast dedicated to the study of flags.
After Sheldon keeps Leonard up all night with an natural disaster drill, Leonard decides that he needs a break from taking care of Sheldon. Hurt, Sheldon invokes clause two-oh-nine of their Roommate Agreement, which minimizes their interaction and 'legally' ends their friendship. After a while, Leonard starts to feel bad for Sheldon because he has no idea how to take care of himself on his own. Sheldon proposes they celebrate 'Leonard's Day' once a year to celebrate all of Leonard's contributions to his life, and Leonard then agrees to reinstate the Roommate Agreement. Meanwhile, Wolowitz is preparing to go on a flight to the International Space Station, and he notices that all of his crew mates have cool astronaut names. He enlists Koothrappali to help him subliminally plant the name 'Rocket Man' into his crew mates' minds, but their plan fails and his crew code name becomes Fruit Loops.
Sheldon is forced to use his two weeks of vacation time from work, so he goes to work with Amy in her biology lab. He feels the tasks she asks him to do are menial and boring, but when she finally relents and asks him to dissect a brain, he cuts his thumb and passes out from the pressure. Bernadette wants Wolowitz to sign a pre-nup before their wedding, so Penny asks Leonard to try and get his feelings on the subject.
Sheldon and Kripke argue over who should get Professor Rothman's vacated corner office, so Leonard, Koothrappali, and Wolowitz devise a contest for them which awards the office to the winner. Unfortunately, the contest centers around a subject which Sheldon and Kripke are equally terrible: sports. Meanwhile, Amy buys Penny a garish painting as a gift to celebrate their friendship, but Penny hates it. She and Bernadette struggle to come up with ideas for how to discreetly dispose of the painting.
Sheldon's routine world is turned upside down when he learns that his longtime barber passed away, so he tries to act like a hippy to match his new shaggy hairstyle. Meanwhile, Wolowitz is sent to Houston for the grueling NASA training program, and he shares his horrific stories with Bernadette via Skype.
Amy gets upset with Sheldon when he breaks his promise to accompany her to her aunt's birthday party and instead opts to join the guys for a weekend-long Star Wars game marathon. After a very lopsided game of quarters, a drunk Penny convinces her to confront Sheldon about it, and he eventually apologizes.
Raj may have found his soul mate after his parents set him up on a date. Meanwhile, Sheldon thinks he's going out of his mind when he begins hearing the voice of Spock (Leonard Nimoy) after he breaks Leonard's "Star Trek" collectible.
When Stephen Hawking comes to lecture at the university for a week, Wolowitz is given the enviable task of maintaining his wheelchair. Sheldon begs Wolowitz to introduce him to Hawking, but Wolowitz only agrees after making Sheldon do a few humiliating tasks, including doing his laundry, cleaning his room, and taking his mother dress shopping.
Bernadette reconsiders marrying Howard after learning about his sexual history.
NASA reschedules Howard's mission, putting the wedding plans in jeopardy. Meanwhile, Leonard says something surprising to Penny in the bedroom.
When Howard and Bernadette decide they want to be married before his NASA launch, the gang rushes to put on a wedding.