List of The Big Bang Theory episodes (season 2)

Penny and Leonard return from their first date while Howard and Rajesh spy on them using a webcam on the corridor. Penny tells Leonard that they should "slow things down", and the guys try to convince Leonard that the date didn't go well. On the other hand, Penny reveals to Sheldon that she lied to Leonard about completing community college, as she is afraid that she isn't smart enough for him. Subsequently, Penny asks Sheldon to keep this as a secret, but this request becomes too hard for him that he makes the drastic decision to move out of his and Leonard's apartment. After staying briefly with Raj, and then Howard, a drugged Sheldon is brought back to Leonard's apartment, where he admits that Penny didn't complete community college. The next day, in an attempt to help her, Leonard offers Penny some pamphlets regarding a local community college, but accidentally mentions that he is fine with dating a girl "who isn't smart". Penny promptly slams the door in his face as Leonard realizes his mistake.

After the guys return from a Renaissance fair and see Penny with her new boyfriend, Eric (Travis Schuldt), Leonard decides to once again pursue a relationship with fellow physicist Leslie Winkle (Sara Gilbert). Thinking that she needs to "slow things down", Leslie agrees to date Leonard, repeatedly assuming the dominant role before conceding that Leonard should assume the "male role". With this new attitude, Leslie goes as far as evaluating Leonard's genetic defects, and talking about procreating children with him. Sheldon explains to Penny that he feels uncomfortable with Leonard dating his arch enemy, who is a "sloppy, arrogant, sub-par scientist". After Penny's advice, Sheldon formally approves the relationship between Leonard and Leslie. However, almost immediately, Leslie and Sheldon start an argument about the validity of loop quantum gravity, which Leslie supports, and string theory, which Sheldon supports. Leslie expects Leonard to back up her beliefs, but when Leonard prefers string theory, Leslie decides to dump him.

Recurring characters: Sara Gilbert as Leslie Winkle

Sheldon becomes distracted from playing Age of Conan by Penny who has been accidentally locked out of her apartment. Along with this event, Penny is frustrated by her failures in life such as not progressing in her acting career, and not having sex for six months. As Sheldon invites Penny into his apartment to wait for the locksmith to open her door, she becomes curious about the game Sheldon is playing, and quickly becomes addicted. In the following days, Penny experiences video game addiction, calls Sheldon at all times for help, and stops going to work to play the game nonstop. After talking with Leslie Winkle, Sheldon realizes that Penny needs to have sex to relieve her stress and regain her regular life. Sheldon then attempts to set Penny up with an online dating service, but fails miserably. In the end, Penny overcomes her addiction when she realizes she accepted to have a virtual date with Howard. Horrified, she promptly stops playing.

Raj is included in People magazine's "30 visionaries under 30 years to watch" list for his discovery of the trans-Neptunian object 2008 NQ₁₇, sparking jealousy from his friends. Despite Leonard's comments that they need to be supportive, fame soon goes to Raj's head: he receives star treatment from Dr. Gablehauser (Mark Harelik), gets an assistant, and even invites Penny to People's reception. After the party, a very intoxicated Raj returns home with Penny, and via webcam introduces her to his parents as his girlfriend; the night ends with Penny leaving and denying being Raj's girlfriend, and with him feeling sick from drinking too much. The following morning, a sober and shy Raj tries to apologize to Penny for his behavior. Although Raj attempts to leave a written note, Penny ultimately forces him to utter a barely audible "Sorry", at which point she forgives him.

The episode includes a cameo appearance from Charlie Sheen; at Penny's restaurant Raj tells a patron that he is going to appear in People magazine, but the patron, Sheen, turns around and says "Yeah? Call me when you're on the cover."

Leonard is working nights on an experiment, resulting in his inability to drive Sheldon to the university. As a result, Sheldon must find another way to get to work. He asks Penny, Howard and Raj for assistance, but they all eject him from their respective vehicles for being annoying. The friends finally stage an "intervention" and tell Sheldon to get a driver's license and learn to drive. After an eventful trip to the DMV, resulting in Sheldon getting his learner's permit, the group sets up a driving simulator for him in the living room. The ensuing practice shows that Sheldon is a horrible driver, but instead of admitting defeat, he simply concludes that he is too highly evolved to drive a car. In the end, Sheldon decides to live in his office at the university, so that he doesn't have to commute. However, Leonard reveals that he actually finished his experiment a week ago and is just enjoying the apartment without Sheldon.

After a lecture by Leonard and Sheldon, a graduate student named Ramona Nowitzki (Riki Lindhome) becomes obsessed with Sheldon, and insinuates herself into his life as a girlfriend/assistant. Ramona proves to be a helpful influence on Sheldon's work, but he becomes tired of her as she prohibits his recreational activities, and forces him to solely concentrate on his research. Eventually Sheldon realizes that he is in "some sort of relationship" with Ramona, and seeks help from Penny and Leonard to get rid of her, without success. Things come to a conclusion when Sheldon reaches a breakthrough in his scientific research, and Ramona suggests they share credit for his discovery. Naturally, Sheldon refuses to share credit, and subsequently kicks Ramona out. In the end, after reading a draft paper of the new theorem, another student approaches Sheldon in the same way Ramona did the first time. The episode ends with Leonard having a bad dream of Sheldon procreating by splitting in two after consuming too much Thai food.

When Penny's cable gets cut, she starts coming over to the guys' apartment to watch America's Next Top Model. However, Sheldon bans her from the apartment for a series of petty infractions, such as touching his food, sitting in his spot, and sending him "email humor" about a cat "who wants to haz cheezburger". After a series of escalating pranks, including Sheldon cutting off Penny's access to WiFi and Penny wrecking Sheldon's "laundry night", Sheldon goes to the extreme of hanging her laundry from a telephone wire outside his apartment. Leonard decides to help Penny end the war, and gives her Sheldon's "Kryptonite": his mother's phone number. Subsequently, Sheldon's mother calls him and forces him to apologize to Penny.

Meanwhile, Howard and Raj try to find the location of the Next Top Model house in Los Angeles. They use all of the technology at their disposal, including a military spy plane and NASA satellites to finally pinpoint it. In the end, they arrive at the house, and posing as cable guys work their way in. The episode ends with cameos of Analeigh Tipton and Samantha Potter of America's Next Top Model, Cycle 11.

A smitten Howard thinks he's found his soul mate, until the woman, Dr. Stephanie Barnett (Sara Rue), meets Leonard. Howard invites Stephanie to drive the Mars rover at the Jet Propulsion Laboratory, but the plan fails when the rover winds up stuck in a Martian ditch. Subsequently, to hide evidence of what he did, Howard destroys all the security tapes and hard drives relating to the mission. Afterwards, Leonard has a hard time deciding how to tell Howard that he has started dating Stephanie. Leonard decides to go to Howard's house to tell him personally, but while there, Stephanie calls Howard and gives him the news. Howard becomes very upset with Leonard, but this only lasts until Stephanie sets up Howard with another friend, and invites him on a double-date. In the end, Howard hears the news on TV that the rover found water on Mars in the bottom of the ditch, but as so much data was "inexplicably" lost no one will know who was responsible for the amazing discovery.

In a minor sub-plot, Sheldon and Raj play a game of Rock-paper-scissors-lizard-Spock for who gets to watch what on TV, but they choose spock. They, along with Howard, play the game for the last dumpling. When they choose spock again, They play it for one of them not holding up spock, but they chose Spock again.

Recurring characters: Sara Rue as Dr. Stephanie Barnett

Sheldon considers Dr. Stephanie Barnett (Sara Rue) the only "tolerable" girlfriend that Leonard has had. As a result, Sheldon tries helping Leonard in his relationship with Stephanie, as otherwise he thinks Leonard will ruin it. However, Leonard doesn't want Sheldon's help as he quickly becomes annoying, even joining the couple when they have a romantic dinner, and when they go to the movies. After believing that Leonard is losing Stephanie, Sheldon attempts to prove Leonard's physical superiority to Stephanie by having him open a jar of white asparagus. However, Leonard shatters the jar by accident and has to get stitches. Following the trip to the hospital, Leonard finds Sheldon hacked his Facebook account, and changed his status to "In a relationship". Leonard worries that this will make him look desperate, but then finds that Stephanie changed her status to "Stephanie Barnett is in a relationship with Leonard Hofstadter". Sheldon then proclaims "Dr. Sheldon Cooper for the win."

Recurring characters: Sara Rue as Dr. Stephanie Barnett

For all practical purposes, Leonard's current girlfriend, Dr. Stephanie Barnett (Sara Rue), has moved into Leonard and Sheldon's apartment, although Leonard doesn't acknowledge this. Meanwhile, Stephanie gives a physical examination to Sheldon who complains about a ringing in his ear. After Penny shows up scantily clad in the guys' apartment, it turns out that neither Leonard nor Sheldon ever told Stephanie about Penny. The morning does not go well following that; Penny proves to Leonard that Stephanie is living with him, and Stephanie buys Leonard new clothes that he ends up hating. Realizing that his relationship with Stephanie is moving too fast for him, Leonard tries to tell Stephanie to move out, but every time he tries she diverts him by initiating sex. Meanwhile, Sheldon sneaks into the hospital to test himself multiple times, as the ringing hasn't stopped. By the time Leonard gets back to the apartment, Stephanie has diagnosed Sheldon with a fake larynx inflammation, and instructs him not to speak at all. Although Leonard and Stephanie are not seen breaking up, evidence suggest they ended their relationship, as Stephanie doesn't appear in the following episodes.

Recurring characters: Sara Rue as Dr. Stephanie Barnett

Sheldon worries about the approaching Christmas holidays and his indecision on what present to get Penny. After deciding on a basket of bath items, he discovers a wide selection of them is available; unsure of what Penny is going to get him and thus what sort of reparations will be appropriate, he buys an entire array to cover all contingencies. Penny's gift—a napkin both autographed and used by Leonard Nimoy, thus containing DNA for Sheldon to clone his own Mr. Spock—is so extravagant that he gives her all the baskets and a hug.

Meanwhile, Leonard meets visiting researcher Dr. David Underhill (Michael Trucco), who is an experimental physicist like he is. Although Leonard envies David for being handsome, charming, cool, and a more successful physicist than he is, he still jumps at David's offer to help him in his research. However, Leonard's joy of hanging out with David is hampered by jealousy when the latter meets Penny, and the two start dating—but soon after, Penny breaks up with David after learning that he recently married.

Note: This episode was selected as an Emmy submission for Jim Parsons, which resulted in his nomination for Best Actor in a Comedy Series.

Howard makes Penny angry and her harsh comments about his love life sends him into depression, and as a result, his friends lose their most important team-mate in an upcoming fighting robot competition. Subsequently, when Sheldon accepts a challenge for a one-on-one fight, pitting their robot, M.O.N.T.E. (Mobile Omnidirectional Neutralization and Termination Eradicator), against "the Kripke Krippler" from their co-worker, Barry Kripke (John Ross Bowie), Leonard tries to get Howard back on their team by sending Penny to apologize to him. Penny goes to Howard's house to apologise, and ends up hearing his entire dating history. After Penny's comforting words, Howard attempts to kiss her. In reaction, Penny punches him in the face. Meanwhile, M.O.N.T.E. is destroyed beyond repair and Penny accidentally sends Sheldon into depression by telling him M.O.N.T.E. was "just a toy robot".

Recurring characters: John Ross Bowie as Barry Kripke

Sheldon hears that Barry Kripke (John Ross Bowie) is in control of an open science grid computer, so he plans to befriend him to get access to the computer. Socially inept, Sheldon has trouble understanding how to make a new friend, and his current friends aren't of much help in this respect. After a trip to the bookstore, Sheldon uses a children's book to help him design a flowchart to get new friends. This "scientific" procedure pays off, and Sheldon goes rock-climbing with Kripke. Afterwards, they are back at the apartment, where Sheldon decides to cut off Raj as a friend to make room for Kripke. However, when Kripke reveals that he does not control use of the grid computer, Sheldon dismisses him and restores Raj's place in the line-up.

Recurring characters: John Ross Bowie as Barry Kripke

Penny is experiencing financial difficulties, which may result in her finding a new place to live. Sheldon, after discussing her difficulties, does the logical thing and lends her some money out of his savings. This leads to an amusing role reversal: while Sheldon does not particularly care about when his loan is repaid, Penny begins displaying neurotic behavior, reacting irritably to anything that hints at her monetary troubles. After Leonard learns about Penny's problems, he tries to help her cut expenses, and in the process also learns that her ex-boyfriend Kurt (Brian Patrick Wade) owes her a big sum of money. Without telling Penny, Leonard and the guys go to Kurt's place to collect the money, but they are unsuccessful. Later, Penny pays Sheldon back, revealing that Kurt actually paid her in turn. However, much to Leonard's disappointment, Kurt never told Penny about Leonard's visit, and instead took the opportunity to get back together with her.

Recurring characters: Brian Patrick Wade as Kurt

Leonard suddenly becomes worried when he learns his mother, accomplished psychiatrist and neuroscientist Dr. Beverly Hofstadter (Christine Baranski), is coming to visit. Upon arriving at the building, Beverly meets Penny, and by the time they reach Leonard's apartment, Penny is in tears after having her childhood issues psychoanalysed. Sheldon, on the other hand, instantly develops a comfortable relationship with Leonard's mother, who turns out to be as strict and logical as himself.

Howard and Raj also meet Beverly, but after learning that Howard still lives with his mother and that Raj can't speak to women, she is quick to declare that they are in an "ersatz homosexual marriage". Later, Leonard goes to talk to Penny, and they both seek comfort in each other, discussing their respective childhood issues with their parents. As they become drunk, Penny and Leonard are about to have sex, but Leonard ruins the moment by using psychology and noting that he is having sex with his mom and she with her dad. Disgusted, Penny throws Leonard out of the apartment.

Recurring characters: Christine Baranski as Dr. Beverly Hofstadter

Note: This was the most watched episode of season 2, with 13.11 million viewers. Christine Baranski was also nominated for an Emmy, for Best Guest Actress in a Comedy Series.

A paintball game leads to a romance between Howard and Leonard's former girlfriend, Leslie Winkle (Sara Gilbert). At the apartment, Penny accidentally shoots the paintball gun at Sheldon's couch cushion, and she and Leonard have a hard time getting it cleaned. Naturally, Sheldon freaks out when he finds out his favorite spot has been damaged. In the end, Sheldon is still dissatisfied with the dry-cleaned cushion, and he team-kills Penny in the next paintball game, causing the others to "surrender, then (go to) Denny's".

Meanwhile, Leslie and Howard start sleeping together, and she is able to secure funds for his work, and even invites him to a trip to the CERN Large Hadron Collider project in Geneva. However, this turns out to be just Leslie's way to control Howard. Although initially reluctant to be in this sort of relationship, Howard later happily accepts his role as a "sex toy/arm candy".

Recurring characters: Sara Gilbert as Leslie Winkle

The guys take a train trip to a conference in San Francisco, where the guest speaker is 2006 Nobel Prize laureate Dr. George Smoot. However, the trip suddenly becomes more exciting when the guys realize Summer Glau (guest starring as herself) of Terminator: The Sarah Connor Chronicles is on board. Leaving his shyness behind while drinking beer, Raj is the first to approach Summer. However, when the beer turns out to be non-alcoholic, Raj reverts to his usual timid self. Howard then takes over, but creeps Summer out with his wild dreams. Leonard eventually gets his chance, but Summer gets off the train before he can introduce himself. Meanwhile, Sheldon realizes he forgot his USB flash drive with a paper he wants to show to Smoot, and he needs Penny to find it in his room and email the paper to him. In the end, Dr. Smoot (guest starring as himself) is not impressed with Sheldon's proposal of research, asking him "With all due respect, Dr. Cooper, are you on crack?"

Penny develops a home-made hair barrette, called the "Penny blossom," which she expects to turn into a profitable homebased business. Sheldon soon points out the flaws in her production process, and helps her develop a better assembly line. Leonard, Howard, and Raj also contribute their own ideas for efficiency, production and marketing. Leonard designs the sales website, and right away they get an order for a thousand blossoms from the "East Rutherford, New Jersey Gay-Lesbian-Bisexual-Transgender Alliance". Despite their initial happiness, they realize Leonard included a one-day rush shipping option in the website, so the quintet has to work all night to fulfill the order. However, the next morning, they discover that the same group doubled their order, leading everyone to quit in exhaustion.

The former tenants of the apartment above Leonard and Sheldon's move out, and are replaced by an attractive blonde woman named Alicia (Valerie Azlynn). Alicia, like Penny, is an aspiring actress, but a more successful one, landing a role in the TV series CSI. Naturally, the men become fascinated with Alicia, and she takes advantage of this by having them labor for her. Penny becomes jealous, thinking that Alicia is supplanting her in the guys' attention. Subsequently, Penny tries to gain her friends back by bribing them with Chinese food, offering to watch Battlestar Galactica, and even memorizing a quantum physics joke. Alicia points out that Penny uses the same routine on the men that she does. Penny becomes enraged and the two have a catfight in the lobby, resulting in a black eye for Penny and ecstasy for Howard. In the end, Penny has reclaimed her men, while Alicia is heard having sex with one of the producers of CSI.

Penny needs a present for her nephew's birthday, so she accompanies the guys to the comic book store. There, she meets the owner, Stuart (Kevin Sussman), who manages to ask her out on a date. Feeling jealous about the situation, Leonard demands that Howard and Raj "take [him] to a bar with women."

Stuart's date with Penny goes well, but they are interrupted by Sheldon who starts an argument about who Batman's successor should be if Bruce Wayne is killed (Sheldon advocates Dick Grayson, Stuart Jason Todd). Sheldon "wins" the discussion when Stuart announces he is tired and needs to leave, whereupon they discover that Penny has fallen asleep from boredom. Meanwhile, at the bar, Raj is the only one who succeeds in getting a girl.

Recurring characters: Kevin Sussman as Stuart

Leslie Winkle ends her "friends with benefits" relationship with Howard, and he becomes depressed. Leonard and Raj decide to take him on a trip to Las Vegas in order to cheer him up. Meanwhile, Sheldon, who has declined to go to Las Vegas, is ready to spend the weekend alone. While in Vegas, a prostitute named Mikaela (Jodi Lyn O'Keefe) approaches Raj, and eventually he and Leonard decide to hire her for Howard. Howard soon realizes she is a prostitute, but nevertheless he thanks Leonard and Raj for setting up this "date". Meanwhile, Sheldon, having forgotten his apartment key, is forced to spend the night at Penny's apartment. In the end, Sheldon thinks he has a better understanding of the term "friends with benefits".

Recurring characters: Sara Gilbert as Leslie Winkle

Howard celebrates the launch of his latest invention, a zero-gravity human-waste disposal system (a "space toilet"), to be used in the International Space Station, by buying all of his friends new comic books only for Howard to discover that he made a mistake that will cause the toilet to fail after a few flushes, and the guys convene to try and fix it. In the end, the astronauts of the ISS go on an unscheduled "spacewalk", suggesting that Howard failed.

While at the comic book store, Stuart (Kevin Sussman) tells Leonard that he is having a second date with Penny and asks for advice. Leonard deliberately avoids Stuart's request, and ultimately gives him bad advice. The next day, Leonard feels guilty and goes to apologize to Stuart. Stuart reveals the date went fine, until he and Penny started kissing and Penny accidentally called him "Leonard."

Recurring characters: Kevin Sussman as Stuart

Sheldon wins a National Science Foundation grant to go on a three-month expedition to the North Magnetic Pole to detect magnetic monopoles and prove the validity of string theory. After some hesitation, he decides to go, and wants Leonard, Howard and Raj to join him; although initially reluctant, as it requires spending three months locked in a cabin with Sheldon, they agree. To prepare for the cold weather, Sheldon asks Penny to give them access to the freezer at the Cheesecake Factory. When Penny learns that Leonard is also going, she is seemingly upset. Later, she gives him a Snuggie as a gift, says she'll miss him, and gives him an unusually long hug. On the morning of the guys' departure, Leonard asks Penny about the long hug, but she doesn't reveal her true feelings for him. Only after they leave does Penny admit, "It means I wish you weren't going".

Note: This is the first episode to feature Sheldon saying "Bazinga", which would later become his catchphrase.