Episode 1 / Big Bang Theory (pilot)
Two brainy best friends, Leonard (series star JOHNNY GALECKI) and Sheldon (series star JIM PARSONS), could tell you anything you want to know about quantum physics -- but when it comes to dealing with everyday life here on Earth, they're lost in the cosmos. Leonard and Sheldon are brilliant physicists, the kind of "beautiful minds" that understand how the universe works. But none of that genius helps them interact with people, especially women. All this begins to change when a free-spirited beauty named Penny (series star KALEY CUOCO) moves in next door. Sheldon, Leonard's roommate, is quite content spending his nights playing Klingon Boggle with their socially dysfunctional friends, fellow Caltech scientists Wolowitz (series star SIMON HELBERG) and Koothrappali (series star KUNAL NAYYAR). However, Leonard sees in Penny a whole new universe of possibilities ... including love.
In the series premiere, when Leonard and Sheldon meet Penny, Leonard is immediately interested in her. But Sheldon feels his friend is chasing a dream he'll never catch
Episode 2 / The Big Bran Hypothesis
After Leonard tries to do a favor for Penny, Sheldon ruins the plan. Leonard volunteers to sign for a package, hoping to make a good impression on Penny. However, when he involves his obsessive-compulsive roommate, Leonard sees his attempt at chivalry go horribly awry.
Episode 3 / The Fuzzyboots Corollary
Leonard is depressed to learn that Penny is dating someone. As a result, encouraged by his friends and fellow scientists, Leonard actually summons the nerve to ask a woman out on a date. Recurring guest star SARA GILBERT ("Roseanne") appears as Leslie.
Episode 4 / The Luminous Fish Effects
After being fired from his job, Sheldon attempts to explore the "dreadful" life beyond physics. But his initial thrill of shopping with Penny devolves over time to him weaving ponchos as a shut-in, forcing Leonard to call on Sheldon's mother, Mary (LAURIE METCALF - "Roseanne").
Episode 5 / The Hamburger Postulate
After sensing that his pursuit of Penny isn't going anywhere, Leonard agrees to an unexpected sexual encounter with fellow scientist Leslie Winkle (recurring guest star SARA GILBERT). The one-night stand throws Leonard and Sheldon deep into uncharted social waters.
Episode 6 / The Middle Earth Paradigm
The socially stunted guys are nervous when Penny invites them to a party she's hosting. But they are ecstatic to find out that it's a Halloween party and enthusiastically don their respective nerdy costumes, which unfortunately fail to impress Penny's brutish ex-boyfriend. Afterwards, Koothrappali experiences the unexpected with a partygoer, Cheryl (ERIN ALLIN).
Episode 7 / The Dumpling Paradox
When Wolowitz seduces Penny's friend and ends up having sex with her in Penny's apartment, Penny sleeps on Leonard and Sheldon's couch. Meanwhile, with Wolowitz occupied, the guys discover that they are without a fourth member of their group. In desperation, they ask Penny to join one of their two-person Halo teams.
Episode 8 / The Grasshopper Experiment
Koothrappali becomes overwhelmed when his parents arrange a blind date for him, but he discovers he actually can talk to a member of the opposite sex after a few drinks. However, complications arise when his beautiful date shows more interest in Sheldon.
Episode 9 / The Cooper-Hofstadter Polarization
Sheldon and Leonard are invited to present their joint findings at a physics conference, but Sheldon refuses. Penny steps in to mediate, but this only deepens the rift between the two best friends.
Episode 10 / The Loobenfeld Decay
After hearing Penny sing, Leonard lies to avoid seeing her perform. Sheldon goes one step further with a more elaborate lie, leading to the appearance of his nonexistent cousin. D.J. QUALLS ("Road Trip") guest stars.
Episode 11 / The Pancake Batter Anomaly
When Sheldon gets sick, Leonard and the boys know what is in store for them and avoid him like the plague; this leaves an unsuspecting Penny to nurse the world's worst patient back to health.
Episode 12 / The Jerusalem Duality
When Leonard and Sheldon meet a 15-year-old physics prodigy, Sheldon is distraught that this boy genius is younger and smarter. In an attempt to restore a sense of purpose, the boys come up with a solution to derail the young prodigy's research.
Episode 13 / The Bat Jar Conjecture
The guys decide to compete in the university's Physics Bowl, but since Sheldon's only focus is to prove his mental superiority, the guys kick him off the team. To take his spot, they enlist his nemesis Leslie Winkle (recurring guest star SARA GILBERT).
Episode 14 / The Nerdvana Annihilation
When Leonard and the guys buy a time-machine prop from a classic 1960 movie, its arrival creates problems for Penny, and she tells the boys that their obsession with comics and toys is "pathetic."
Episode 15 / The Pork Chop Indeterminacy
Leonard, Wolowitz, and Koothrappali all become smitten with Sheldon's twin sister, Missy (guest star COURTNEY HENGGELER), and Leonard devises a plan to eliminate his rivals. The plan backfires, however, when Sheldon declares he will decide whom his sister dates.
Episode 16 / The Peanut Reaction
When Penny learns that Leonard doesn't celebrate birthdays, she attempts to throw him a surprise party; she is sidetracked by Sheldon, however, who unexpectedly gets to live out one of his greatest fantasies at an electronics store.
Episode 17 / The Tangerine Factor
When Penny breaks up with her boyfriend, Leonard finally gets up the courage to ask her out on a real date. When she says yes, each seeks the "wisdom" of a reluctant advisor -- Sheldon.