The series is about two socially inept prodigies in their 20s who are living across the hall from a beautiful girl with a questionable intellect. When one genius falls for the girl, the other tries to discourage his interest because he believes his friend is chasing a dream he'll never catch.
On October 19, 2007 the show was picked-up for a full 22 episode season, the first comedy of the 07/08 year. 
Several of Johnny Galecki's co-stars from his previous sitcom, Roseanne guest star in the series, including Sara Gilbert as Leonard's secondary love interest (a similar role to the one she played on Roseanne) and Laurie Metcalf as Sheldon's mother.
The sitcom itself takes place in Pasadena, CA, but the audience very rarely sees the characters venture outside of the apartment building. Most of the action takes place in Leonard and Sheldon's apartment, Penny's apartment, the hallway adjoining the two apartments, the staircases leading up to and above the hallway, and the lobby of the apartment building. It seems that the elevator is always out of order and that whenever we see Leonard and Sheldon traveling from outside their building to inside their apartment, the audience is able to see them as they walk up three flights of stairs. Also, the white boards that Sheldon and Leonard use for jotting down scientific theories and hypotheses hang in their apartment and have different information on them in each episode.
The show was criticised very early on for their presumed use of canned laughter, though the show is taped in front of a live studio audience.
The title of the show is a double entendre, both describing the scientific theory of the big bang that the geniuses portrayed in the show often discuss and the term 'bang', sexual slang for 'to have sex with', describing the presumably one-sided sexual tension between one genius and beautiful girl with whom he falls in love.
The band Barenaked Ladies sing the theme song, which describes the progress the human race has made since the dawn of time. During the theme, visual illustrations of such progress are shown and as the last line of the theme is played, it cuts to a scene of Leonard, Sheldon, Howard Wolowitz, and Rajesh Koothrappali gathered around Penny while they eat take-out, with the title card superimposed over them.
Leonard is a physicist from Caltech that rooms with Sheldon in their rundown apartment building, in which the elevator always seems to be out of order. He keeps a whiteboard in their living room on which he works on theoretical physics. While Sheldon's genius seems to be focused primarily in math and science, Leonard is insinuated to be proficient in literature and history as well as the sciences; he is able to solve a woman's crossword puzzle in seconds, the answers being mostly literature based, e.g Vladimir Nabokov. It is also insinuated that Leonard's IQ is not as high as Sheldon's; Sheldon scoffs at Leonard's white board theories as common enough to be "scribbled on the wall of every men's room at MIT". Leonard feels at home with his genius colleagues, but wants more socially. He is reluctant to let Penny know about his recreational activities (such as KlingonBoggle) and is probably reluctant to even partake in such activities in the first place, because although he secretly likes them, he doesn't like to be seen as a nerd. He also dislikes his friends' habit of rattling off different trivial facts, something to which all the other geniuses enjoy both listening and participating. Sweet but sarcastic, Leonard also seems to be the most well-adjusted one of the group; he is quick to try new things apart from his nerd friends and, between he and Sheldon, is the only who feels that he "should do something" when Penny begins to recount her recent break up. Despite his attempts to reject his nerd status, he certainly is one, with his Star Wars hygiene products and his ability to blurt out the wrong things in front of women, such as when he's trying to invite Penny to lunch and mentions that curry is a natural laxative. He falls head over heels for Penny within seconds of spotting her and while at first it seems like lust rather than love, by the end of the pilot episode he has already announced to Sheldon, "our babies will be smart and beautiful." Sheldon, however, is quick to add, "not to mention imaginary."
Leonard wants to win a Nobel prize someday (though Sheldon doubts he'll ever achieve it).
Leonard is a huge Superman fan, so much so that he owns all of the Superman films and more than 2600 Superman comics.
Leonard once dated a, presumably Korean, girl named Joyce Kim. He claims to have broken up with her, but Sheldon reveals that the end of their relationship was because she actually defected to North Korea.
Leonard is commonly seen wearing "Geek Ware" (clothes that are scientifically inspired) such as, in episode 103 where he can be seen wearing a shirt with a "Paradox" monocle.
Leonard and Sheldon have a combined IQ of 360, making the average IQ 180.
Leonard and Sheldon own Dell XPS computers.
Leonard can play the cello.
Leonard's middle name is Leakey.
Leonard and Sheldon are named after actor and producer Sheldon Leonard.
Sheldon is Leonard's best friend, roommate, and colleague, who appears to have OCD, beginning with suspicion of it in episode 102. He's a physicist like Leonard and also keeps a white board in the living room for scientific theories. Unlike Leonard, he embraces his genius fully and doesn't mind sharing details of his life that Leonard finds embarrassing, such as their main weekend activity being Klingon Boggle tournaments until 1 AM. It's insinuated that Sheldon's genius is concentrated fully on science and that his IQ is higher than Leonard's; he tends to question Leonard's theories and has a larger background full of behavior common to child prodigies. When he was a child, he conducted experiments involving the height of stairs and deduced that if a step on a staircase is 2 millimeters off or more, a person is always likely to trip (this resulted in his father breaking his clavicle). At age 14, he dabbled in lasers and this resulted in his being sent to boarding school; it is heavily implied in a CBS promotional clip that the government took Sheldon's laser away and hushed the college professor who witnessed the experiment. Sheldon graduated high school very early; he asks Penny, in complete disbelief, "It took you four years to get through high school?!" He told Penny he was eleven years old when he was going to college. Sheldon also has common qualities associated with being a prodigy, such as an inflated ego, social ineptness, and an inability to identify emotionally with others. He fails to understand not only the simplest sarcastic jokes made by Leonard, but also regards Penny's sadness over her break up with blatant disgust. That isn't to say Sheldon doesn't have a conscience; he's simply more likely to do what's scientifically ethical (not donate to a "high IQ sperm bank" because his sperm won't guarantee high IQ offspring) rather than morally and socially ethical (break into Penny's home to organize her things). Sheldon also remains un-swayed by Penny's beauty. He also has symptoms of obsessive compulsive disorder and Asperger syndrome; when he finds Penny's apartment messy, he sneaks in during the night and cleans it up because he can't stand being in the same hallway with something so disastrous. Also, he refuses to sit anywhere other than his designated spot on the couch. He is calculating and cynical, believing that Leonard is only setting himself up for disappointment as he continues to crush on Penny.
Sheldon, according to Leonard, masturbates quite often.
Sheldon has two siblings, one of whom is a sister who hostesses at Fuddrucker's.
Penny is Leonard and Sheldon's new neighbor across the hallway and is, as described by Wolowitz, a "cheesecake-scented goddess". This description is fairly accurate, considering Penny works for minimum wage at the local Cheesecake Factory. She is also writing a movie about a girl from Lincoln, Nebraska, who travels to Los Angeles to become an actress but ends up working at the Cheesecake Factory instead. When Leonard asks if the movie is autobiographical, she replies no because she's from Omaha, Nebraska. The reason Penny is in the apartment building in the first place is because she recently split with her boyfriend of four years, Kurt, and was thrown out of their place. She says she still loves him but wants custody of their television set. The shower in her apartment is currently broken, much to Leonard and Sheldon's friends' delight (she now uses Leonard and Sheldon's shower and tends to wear nothing but a towel). She is very aware of her neighbors' nerd status but remains friends with them anyway, revealing her kindly nature. She's either ignorant of or all too aware of her effect on the boys, because within minutes of meeting them, she asks Leonard to retrieve her television set from ex-boyfriend's apartment, which he immediately sets out to do. It is not known if Penny returns Leonard's affections. She is messy and disorganized but appears to like it that way. Whenever Penny goes on a rebound relationship, she picks up an attractive man and then has 36 hours of meaningless sex over the course of a weekend; afterward, the relationship deteriorates.
Wolowitz is an engineer who often hangs out at Leonard and Sheldon's apartment. He designed a satellite that is currently circling Jupiter's orbit taking high-res photos. He doesn't show signs of discontentment like Leonard, but rather is confident enough to try to woo Penny with his knowledge of languages, of which he knows six. Most of the languages were revealed in only the matter of one episode; in revealed order, they are English, French, Mandarin, Russian, Arabic, and Farsi. Hitting on Penny in different tongues has become a regular occurrence; one which angers Leonard greatly. Wolowitz also knows Klingon because as Sheldon stated, they all played Klingon Boggle together.
Wolowitz is a fan of Stephen Hawking and likes to watch tapes of his lectures, preferably ones prior to 1985, "before he became a 'creepy' computer voice" (Hawking lost his voice due to a tracheotomy performed after a life-threatening bout of pneumonia and now communicates using a voice synthesizer).
Wolowitz spends a large amount of time playing MMORPGs like World of Warcraft; additionally, on one of these RPGs, he has a tiger companion named Buttons.
Koothrappali is Wolowitz's best friend and yet another genius. He has a thick Indian accent and like Wolowitz, a love for Stephen Hawking. He approaches women in the exact opposite way of Wolowitz: he is so shy that he is unable to speak to them. When Penny speaks to him, he voices a lengthy inner monologue, sometimes reminiscing of his native customs and even singing.
Raj owns a 17" Macbook Pro.
Raj has thought about someday marrying an Indian woman so she can sing lullabies from his native land to his future children.
Kurt (Brian Wade) (two episodes): Kurt is Penny's ex-boyfriend. He is a typical intimidating bully and not only has custody of his and Penny's old apartment, but their old television set as well, which Penny wants very badly. When Leonard and Sheldon try to retrieve the TV for her, he takes their pants and forces them to go home pantsless. Penny left him because he cheated on her.
Leslie Winkle (Sara Gilbert) (two episodes): Leslie is another physicist who works in the same lab as Leonard. Leonard tries to ask her out because he considers her to be in his league (indeed, she is his female doppelganger, glasses and all) and they decide to kiss to see if they have chemistry. Leslie feels no sparks and no date occurs. Leslie usually doesn't shave her legs and only started to recently to imply sexual availability. Leslie plays in a string quartet and appears to be interested in Leonard.
Mary Cooper (Laurie Metcalf) (one episode): Mary is Sheldon's long suffering Southern mother. She has given birth to two children besides Sheldon, both apparently Sheldon's intellectual opposites. Mary herself isn't book smart but is very wise; she appears to be an extremely good mother and is the only one who has ever been able to control Sheldon (besides the government). Sheldon's father has long since passed on and he is mentioned as being incredibly foolish. His death may have resulted from one of Sheldon's dangerous childhood experiments, of which he usually bore the brunt; of course, this is merely speculation.
In the following summary, "rating" is the percentage of all households with televisions that tuned to the show, and "share" is the percentage of all televisions in use at that time that are tuned in. "18-49" is the percentage of all adults aged 18-49 tuned into the show. "Viewers" are the amount of viewers, in million, watching at the time. "Rank" how well the show did compared to other TV shows aired that week.
Unless otherwise cited, the overnight rating, share, 18-49 and viewing information come from Your Entertainment Now. The weekly ranks come from The Programming Insider.
Two brilliant ("a combined IQ of 360") young physicists, Leonard and Sheldon, are the kind of "beautiful minds" that understand how the universe works. But their PhDs don't help them interact with
people, especially women.
The Big Bang Theory: Episode 2 -
The Big Bran Hypothesis
Sheldon and Leonard go over to Penny's apartment to drop off a package that came for her. Sheldon is deeply disturbed when he sees how messy and disorganized her apartment appears. Later that night,
when Leonard is sleeping, Sheldon sneaks...
The Big Bang Theory: Episode 3 -
The Fuzzy Boots Corollary
On the date Penny mentions the man Leonard had seen was not her boyfriend, and that she has a habit of having weekend long flings to get over relationships. Excited, Leonard begins to become more
confident but after accidentally hurting...
The Big Bang Theory: Episode 4 -
The Luminous Fish Effect
Sheldon is fired from his job as a physicist at the university when he insults his (in his opinion) less-gifted new boss Dr. Eric Gablehauser (Mark Harelik). His change of circumstance sends him into
a downward spiral of depression in which...
The Big Bang Theory: Episode 5 -
The Hamburger Postulate
When Sheldon comes home and finds a necktie on Leonard's door, he runs to Penny’s apartment to ask her what the necktie means. Penny tells him and seems slightly disappointed, though she does
congratulate Leonard later on. Leonard tries to...
The Big Bang Theory: Episode 6 -
The Middle Earth Paradigm
The guys are invited to Penny's Halloween party, first turning it down as they 'don't dance' but changing their mind when costumes are mentioned. Once at the party only Wolowitz and Leonard attempt
to socialize; with mixed success. Penny's...
The Big Bang Theory: Episode 7 -
The Dumpling Paradox
Penny's slutty 'friend' Chrissy (Brooke D'Orsay) from Nebraska is in town, and ends up being seduced by Wolowitz. The pair take over her apartment causing her to sleep on Leonard and Sheldon's couch
for the night. With Wolowitz gone Penny...
The Big Bang Theory: Episode 8 -
The Grasshopper Experiment
Koothrappali is overwhelmed after his parents arrange a blind date with a girl from his childhood, as he can not talk to attractive members of the opposite sex. Penny takes up bar tending for some
extra money; and as she practices mixing...
The Big Bang Theory: Episode 9 -
The Cooper-Hofstadter Polarization
Cleaning up after an internet experiment Leonard discovers a letter in the trash can. He and Sheldon had been invited to present some research they had done together at a physics conference. However
Sheldon does not believe in such things...
The Big Bang Theory: Episode 10 -
The Loobenfeld Decay
Penny was given a part in Rent, though after hearing her sing "Out Tonight" extremely off-key ("What the hell is that?" "I don't know, but if cats could sing...they'd hate it too). Leonard lies to
avoid seeing her performance so he won't...
The Big Bang Theory: Episode 11 -
The Pancake Batter Anomaly
Penny returns from visiting family in Nebraska, and as she picks up her mail from Leonard she mentions it was a bad trip because 'everyone got sick'. Sheldon freaks out as according to Leonard 'he's
a bit of a germaphobe'. Sure enough...
The Big Bang Theory: Episode 12 -
The Jerusalem Duality
Sheldon is overcome with I.Q. envy when he meets a smarter 15-year-old science prodigy named Dennis Kim (Austin Lee). Losing faith in his own research, he annoys his circle of friends and colleagues
by criticizing their work (prompting them...
The Big Bang Theory: Episode 13 -
The Bat Jar Conjecture
The guys decided to compete in a physics bowl but find Sheldon's showboating too much and kick him off the team, telling him "You're annoying and no one wants to play with you anymore". Upset,
Sheldon vows to form his own team and compete...
The Big Bang Theory: Episode 14 -
The Nerdvana Annihilation
Leonard buys a replica time machine online believing it to be a miniature... though it turns out to be full-sized. As the boys move it into the apartment they accidentally make Penny late for work
causing her to lash out at Leonard for his...
The Big Bang Theory: Episode 16 -
The Peanut Reaction
When the boys dine at Penny's restaurant she learns Leonard has never had a birthday party, as his parents didn't believe in celebrating anything but achievements. She decides to throw him a surprise
party, and blackmails Sheldon to join...
The Big Bang Theory: Episode 17 -
The Tangerine Factor
Penny breaks up with her boyfriend after he posted intimate details about her on his blog. Leonard goes to comfort her and in doing so accidentally convinces her to get back with her boyfriend. As
Leonard laments the situation to the guys,...
The Big Bang Theory: Episode 1 -
The Bad Fish Paradigm
Penny and Leonard return from the restaurant while Howard and Rajesh spy on them, using a webcam on the corridor connected to a laptop. They try to convince Leonard that the date wasn't working
well: they think that when Penny said "let us...
The Big Bang Theory: Episode 2 -
The Codpiece Topology
The guys return from a Renaissance Fair and see Penny with a new guy, Eric (Travis Schuldt), prompting a jealous Leonard to pursue a relationship with fellow physicist Leslie Winkle. All the guys as
well as Leslie humiliate Leonard for his...
The Big Bang Theory: Episode 3 -
The Barbarian Sublimation
Sheldon becomes distracted from playing Age of Conan by Penny who has accidentally attempted to use her car keys to open her apartment door. Sheldon learns that Penny is frustrated by her failures in
life specifically her not getting into...
The Big Bang Theory: Episode 8 -
The 21-Second Excitation
The guys camp out in line to see a print of Raiders of the Lost Ark with 21 extra seconds and are outraged to see Wil Wheaton and his friends
being allowed to skip the line. Penny and Bernadette invite Amy to their girls night.
The Big Bang Theory: Episode 16 -
The Cohabitation Formulation
Bernadette tells Howard that it's time to choose between moving in
with her or staying with his mother. Leonard discovers Raj's sister Priya is back in town and reconnects with her, and Amy tries to comfort Penny when she finds out.
The Big Bang Theory: Episode 19 -
The Zarnecki Incursion
When Sheldon has his World of Warcraft account hacked and his character stolen from, the guys team up to discover
out who did it. Meanwhile, Bernadette and Amy try to help Penny deal with Leonard's continuing relationship with...
The Big Bang Theory: Episode 22 -
The Wildebeest Implementation
Amy recognizes a dinner invitation to be a chance for Bernadette to
spy on Leonard and Priya for Penny. Meanwhile, a lonely Raj considers taking experimental medication to cure himself of his inability to talk to women while sober, and...
The Big Bang Theory: Episode 14 -
The Beta Test Initiation
Leonard and Penny create lists of 'bugs' in their relationship in an
attempt to strengthen it. Meanwhile, Sheldon and Amy create a YouTube video series called Fun with Flags, and Raj develops an unhealthy attachment to Siri on his...