The Big Bang Theory season 3

When the men return home from their expedition at the Magnetic North Pole, Penny reveals her feelings for Leonard and kisses him when he visits her. Both are interrupted when Sheldon is humiliated on learning that Leonard, Howard and Raj tampered with one of his experiments, and retreats to his mother's home in Galveston, Texas. When Leonard arrives in Texas, it is Mary who convinces Sheldon to forgive him by disagreeing with his views on evolution. Leonard and Penny then finally have sex, but concede that things are now weird between them.

Leonard and Penny vacillate between friendship and a romantic relationship, struggling to resolve the awkwardness. In an attempt to feel comfortable and talk about a solution, they end up getting drunk in her apartment and throwing up all night. Sheldon and Howard stake their most valuable comic books on a bet over the species of a cricket they hear, enlisting a depressed entomologist to settle the bet. After Sheldon loses the bet, he meets Penny and after talking to her about Leonard, assures her that she and Leonard can always go back to being friends. After she explains this to Leonard, the two agree to do so, but they find that neither wants to and have sex again.

Penny's bed in her apartment breaks, forcing her to sleep at Leonard and Sheldon's apartment for a few days. Sheldon tries to condition Penny, rewarding with chocolate what he considers to be "correct" behavior. Howard and Raj pretend to be goths to try to meet girls at a goth club, but chicken out when the girls persuade them to get tattoos.

Raj's research on trans-Neptunian objects comes to a dead end and he faces deportation to India unless he gets a new job. Sheldon offers Raj a chance to work for him. Raj accepts but disagreements ensue. Eventually, Sheldon apologizes to Raj, conceding that his theory is correct but refusing to give up on his own. Howard tries to hang out with Leonard and Penny, but intrudes into their relationship.

Howard reminds Leonard of their girlfriend pact: one of the two men (Leonard) who has a girlfriend (Penny) must have her set the other man (Howard) up on a date with her friend. After prodding by Leonard, a reluctant Penny sets up Howard with her friend Bernadette, though they find they have little in common. However, the two bond over their respective relationships with their mothers. Sheldon enters a collectible card game tournament, with Raj as his partner, to compete against his rival Wil Wheaton, who inadvertently humiliated Sheldon when he was a child.

Leonard attempts to fit in with Penny's football-loving friends, despite initially knowing nothing about the game. However, it is Sheldon who surprises him with a large amount of football knowledge (forced on him by his father in his childhood), which Leonard later brings to Penny's football party. When Leonard refuses to fly kites with Sheldon, upsetting him as he never got to experience a normal childhood, Penny feels bad and allows Leonard to go, especially after pointing out how he is boring everyone at the party. Howard upsets Raj by repeatedly abandoning him during activities to pursue women.

When Penny announces that her guitarist friend, whom she dated twice, is going to sleep on her couch for the next few days, Leonard is upset that she did not ask him first. Sheldon is upset by reminders of his quarreling parents as Leonard and Penny fight, Raj argues with his parents and Howard argues with his mother.

Sheldon, despite never having driven a car before, is forced to drive Penny to the hospital after she slips in the shower. Leonard, Raj and Howard go on a camping trip to watch the Leonid meteor shower, but all succumb to the effects of "magic" cookies given to them by Deadhead campers nearby, causing them to blurt out weird secrets including Howard losing his virginity to his second cousin and they forget about the meteor shower.

After three successful dates, Howard reflects on the future of his relationship with Bernadette. He surprises her with a last-minute proposal, which she declines as it is much too soon for either of them. However, he wins her over by performing a cheesy love song, which she finds as romantic. A prank fight between Sheldon and Kripke begins when the latter severely embarrasses the former on National Public Radio's talk show Science Friday by filling Sheldon's office with helium, giving him a high-pitched voice. It ends with Sheldon dropping foam from the ceiling into Kripke's office, but accidentally does it when President Siebert and the Board of Directors are there to see Kripke's progress on one of his experiments. Afterwards, a pre-recorded message from Sheldon plays from Kripke's computer, explaining that Sheldon was responsible and that Leonard and Raj were involved.

Howard becomes jealous of Leonard after Bernadette shows an interest in his work, causing friction in the relationship of all three. Penny becomes jealous over not being able to talk to Leonard about his job, and asks Sheldon to teach her about physics to mixed results.

At Christmas, Leonard's mother visits, much to Leonard's horror and Sheldon's delight. Penny is upset to learn that Leonard has not told her about his mother's visit, nor his mother about their relationship, while Leonard learns that Sheldon appears to know more about his parents' lives than he does.

Penny is upset when Leonard derides her for taking advice from a psychic. Awkward situations ensue as Sheldon and Raj date two women they met at a university mixer, Raj having bribed Sheldon to be his wingman.

Sheldon fears for his safety after his and Leonard's apartment is burgled, and takes various measures to try to increase his security.

After becoming obsessed with a physics problem, Sheldon decides to take a menial job, comparing himself to Albert Einstein, who conceived his theory of relativity while working at the Swiss patent office. Meanwhile, Raj feels left out when Leonard, Penny, Howard and Bernadette go on a double-date at a roller skating rink.

Guest Star: Yeardley Smith as Sandy.

When Leonard gets two tickets to a conference in Switzerland to visit the Large Hadron Collider, Sheldon becomes dismayed when Leonard plans to take Penny instead of him for a Valentine's Day skiing trip. After failing to convince Leonard to change his mind, Sheldon tries to convince Penny to drop out, which leads Leonard to terminate his friendship with Sheldon. However, when Penny has caught the flu, Leonard changes his mind and allows Sheldon to go, only to find out he is sick as well, causing him to take Raj. It is later revealed that Sheldon caught it from Penny when he hugged her.

Sheldon tries to meet Stan Lee at a comic book signing, but is stymied by having to appear in court for driving too slow and running a red light while taking Penny to the hospital in the episode "The Adhesive Duck Deficiency", and enlists Penny to help him win the judge's favor.

Sheldon, Raj, Leonard and Howard acquire a collectible casting of the One Ring, which turns out to be a prop from The Lord of the Rings film trilogy. The group is conflicted on what to do: Leonard wants to send it back to Peter Jackson, Raj wants to sell it and use the profits to buy a jet ski, and both Howard and Sheldon want to keep it for themselves. Leonard gives it to Penny for safekeeping, but Sheldon attempts to steal it while she sleeps and she punches him in the nose. The fight escalates at lunch at the university when all of the guys grab the ring and begin a holding contest, with the last one holding on the keeper of the ring. They somehow walk out the door of the university, get in a car and drive home. Leonard drops out when Penny tempts him with sex. The remaining three try several mind tricks on each other to give up the ring such as attacking their mothers or trying to make the other go pee, which works and Sheldon and Howard have to somehow urinate without letting go of the ring.

In the end, the guys fall asleep on the couch, with no one holding the ring. When they wake up, Leonard tells them he sent it back to Peter Jackson and berates them for their childish behavior and that they would take the ring over having friends, since most likely whoever got the ring would have been made an outcast by the others. However, it is revealed Leonard secretly pocketed the ring and is being kept in a shoe box in his room. Sheldon attempts to steal it again while Penny and Leonard were sleeping, but Sheldon and Leonard get into another argument making Penny leave the apartment.

The characters try to help Sheldon overcome his fear of public speaking in time for him to accept a science award. He is still nervous until Penny gets him drunk, causing him to spiral into a meltdown on stage which quickly goes viral.

Leonard and Penny reflect on their relationship after Penny does not reciprocate when Leonard tells her he loves her. At a bowling match with Stuart and others, including Sheldon's rival Wil Wheaton, the two question their future together, but Wheaton may also be interfering.

Sheldon tries to maintain his friendships with Leonard and Penny, now that they have broken up, as they consider whether they can still be friends with each other.

Dr. Elizabeth Plimpton, a cosmological physicist from Princeton University, accepts Sheldon's invitation to stay over at the apartment during her visit to Caltech. However, she instead takes an immediate interest in Leonard, and the two end up sleeping together, inciting jealousy from Penny. However, it quickly becomes clear that Leonard and Elizabeth both have very different ideas about the nature of their relationship, when she sleeps with Raj and tries to convince Leonard and Howard (who has just broken up with Bernadette) to engage in a foursome with them.

After Leonard and Sheldon have a fight about the thermostat temperature, the former takes shelter in Penny’s apartment. While he is there, he talks about how he originally met Sheldon in 2003. Leonard saw an ad about the offer of being a roommate to someone and took it because of its reasonable rent, but is warned by other tenants of Sheldon’s eccentricity. After passing some trials and signing to the Roommate Agreement, Leonard moves in. He clashes with Sheldon after he interrupts sex with a girl Leonard brought home and is constantly frustrated by his behavior. He invites his friends, Howard and Raj, to come use some genuine rocket fuel (that he had after working on a top secret government project) to power a model Howard brought, but Sheldon consistently warns him he got the formula wrong to convert the rocket fuel into something able to be used for the model. Leonard ignores Sheldon’s warnings and continues with the formula, but it begins smoking and they try to take it outside via the elevator, but Sheldon grabs the formula and pushes Leonard out of it moments before the rocket fuel explodes, destroying the elevator and revealing why it is always out of order. After the story, Penny convinces Leonard to reconcile with Sheldon, and the former does go over back to the apartment to apologize, but inevitably another argument arises when Leonard tries to watch Babylon 5, a show Sheldon dislikes.

Penny introduces her new boyfriend Zack to the other characters as they set up an experiment to bounce a laser off the moon using one of the laser reflectors left by the Apollo program, but grows disappointed with him as she watches him interact with the group. She later breaks up with him and drunkenly rants at Leonard for ruining her ability to tolerate idiots before proceeding to engage in sex with him, leading Leonard to hypothesize that he too can initiate drunken sex, which he tests throughout the episode. Meanwhile, Raj and Howard create a profile for Sheldon on a dating website without his knowledge, which to their surprise matches him with a woman named Amy Farrah Fowler. The two force Sheldon to meet Amy, and are surprised when Sheldon and Amy have a lot in common, ranging from their aversion to soiled hosiery, to their overbearing mothers.