Episode 01: Pilot
Leonard and Sheldon are trying to make some extra cash by visiting a sperm bank for geniuses, but they chicken out and go back to their apartment. When they get home they notice that a very beautiful new neighbor, Penny, has moved in across the hall. They befriend her in the best way they know how: by inviting her over for day-old Indian food. The boys' best friends, Wolowitz and Koothrappali, stop by, and they're equally as challenged with girls. Penny asks the boys for a favor, and they're willing to do whatever she needs. Even if it means going to her ex-boyfriend's place to retrieve her television. Unfortunately for Penny, they come back without the tv, and even more unfortunately for Leonard and Sheldon, they lose their pants as well.
Episode 02: 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 into Penny's apartment to clean and organize it. Leonard catches him and tries to stop him, but Sheldon is obsessed. When Penny wakes to find what Sheldon has done she is furious.
Episode 03: The Fuzzy Boots Corollary
When Leonard sees Penny kissing a guy in front of her apartment door he is devastated. The guys convince him to ask someone out at work, but when he tries and is brutally turned down by a geeky girl, he is even more depressed. Wolowitz conspires to take them all to salsa class at the community center, where he meets all of his conquests. The women there are all 85 and up or 6 feet tall. Leonard finally decides to ask Penny out to dinner and just when he thinks he is making progress, she tells him that it is too soon for her to start dating again.
Episode 04: The Luminous Fish Effect
Sheldon is fired from his job as a physicist at the university when he blows up at his boss who he thinks is not a smart as he is. His change of circumstance sends him into a downward spiral of depression which the boys try to cure by sending for Sheldon's mom. When his mom arrives for a visit, the boys realize his mom is completely the opposite of what they expected. They would never have guessed Sheldon was related to her as she is simple and sweet. But the visit works, and Mom is able to successfully get Sheldon back on track.
Episode 05: The Hamburger Postulate
While eating at the restaurant where Penny works, Leonard and Sheldon run into Leslie, a fellow scientist who works with Leonard. Leslie suggests Leonard play in her string quartet, but what she really wants is to get up close and personal on another level. When Sheldon comes home and finds a necktie on Leonard's door, he drops in to Penny's apartment to her to ask what the necktie means. When Leonard finds out Sheldon has told Penny about his rendezvous with Leslie he is furious. Fearing an imagined love triangle between the two women and himself, Leonard is disappointed to realize all Leslie was interested in was a one-time event.
Episode 06: The Middle Earth Paradigm
The guys go to Penny's Halloween party. Leonard ends up getting humiliated by Penny's hunky ex-boyfriend while Sheldon, Wolowitz and Koothrappali try desperately to make a mark on the social scene but with limited success. After the party, Penny is a little drunk and sad after seeing her ex and ends up kissing Leonard.
Episode 07: The Dumpling Paradox
Penny's promiscuous friend comes to town and Wolowitz hooks up with her. This leaves the guys without their fourth wheel. Sheldon goes a little crazy because his world has been thrown off, and Penny has to stay with the guys in their apartment. With Wolowitz preoccupied, the guys discover they are without a fourth member of their group, and in desperation, they ask Penny to join their Halo team.
Episode 08: The Grasshopper Experiment
Koothrappali is overwhelmed after his parents arrange a blind date, but he discovers he actually can talk to the opposite sex after a few drinks; however, complications set in when his beautiful date is more interested in Sheldon. Penny has started her new job as a bartender so Koothrappali decides to have the date at her restaurant, but he blows it and Sheldon ends up getting the girl.
Episode 09: The Cooper-Hofstadter Polarization
When Sheldon and Leonard are invited to present their joint findings at a physics conference, Sheldon refuses. But when Penny steps in to mediate, it only deepens the rift between the two best friends.
Episode 10: The Loobenfeld Decay
The guys are invited by Penny to see her in a performance of Rent, but they know how horribly she sings, so Leonard lies to get out of it. But Sheldon is so disturbed by the lie, that he makes up a very intricate new lie, which he thinks is better. They tell Penny that Sheldon has a cousin who is a drug addict, and they have to do an intervention in Long Beach. They invite over a fellow physicist, who minored in theater in college -- to be the cousin. Penny comes over, and falls for the whole thing.
Episode 11: The Pancake Batter Anomaly
When Sheldon comes down with the flu, all the guys escape knowing what he is like when he is sick. Sheldon then turns up at Penny's restaurant asking for soup. She ends up leaving work, and taking care of him.
Episode 12: The Jerusalem Duality
In this episode a 14 year old wunderkind is introduced to the physics lab where Sheldon and Leonard work. Sheldon is threatened by this boy, because he can take Leonard's title of youngest genius away. Sheldon suffers a crisis of confidence, and his friends try to console him. In an attempt to restore his sense of purpose, the boys come up with a solution to derail the newcomer's research.
Episode 13: The Bat Jar Conjecture
The guys engage in the University Physics bowl. But Sheldon is so obnoxious, he is kicked off the team. So he forms his own team with the janitor and the cafeteria lady, and he goes up against his buddies. Leslie Winkel played by Sara Gilbert returns in this one to join Leonard's team and kick Sheldon's butt.
Episode 14: The Nerdvana Annihilation
When the guys buy the old time machine prop from the movie, Penny accuses them of acting like children and tells them to grow up. This prompts Leonard to get rid of all of his nerdmorabilia, as he realizes that girls like Penny don't end up with guys who have time machines. But when Leonard sees Penny on a date with a gorgeous hunk, he decides to keep his things.
Episode 15: The Pork Chop Indeterminacy
Sheldon's twin sister comes into town, and she is a complete knockout. Leonard, Wolowitz, and Koothrappali go crazy for her and compete for her attention. When Leonard devises a plan to eliminate his rivals, it backfires when Sheldon declares he will decide whom his sister dates.
Episode 16: The Peanut Reaction
The gang finds out that Leonard's birthday is coming up. Penny wants to throw him a party, but she finds out that his family never celebrated his birthdays. Penny decides to throw him a surprise party and enlists Sheldon to help her out. Wolowitz participates by taking Leonard to the emergency room with him, under the pretense that he ate a peanut and is having a massive allergic reaction. By the time they make it back the apartment, the party is over. Penny feels terrible and gives Leonard a big kiss.
Episode 17: The Tangerine Factor
Penny finally breaks up with her boyfriend, giving Leonard the courage he needs to ask her out on a real date. She agrees to the date. However, before they actually go out, they both seek the "wisdom" of reluctant advisor, Sheldon, on separate occasions. The Tangerine Factor is the first season finale of "The Big Bang Theory."