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 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.

Note: This episode marks the first appearance of future regular cast member Melissa Rauch as Bernadette Rostenkowski and also marks the first cameo appearance of Wil Wheaton on the show.

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, who 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.

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. After Penny punches Sheldon on the nose, fights and deceptions ensue as the four try various means to prevent the others from possessing it, as Penny observes in disbelief.

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 on 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.

Taking shelter in Penny's apartment after an argument with Sheldon, Leonard tells Penny how he and Sheldon met in 2003. He describes the trials he underwent in order to become Sheldon's roommate, which reveal many aspects of the latter's character, and in the process reveals how their building's elevator came to be out of order.

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 sock aversion to their overbearing mothers.

Note: This episode marks the first appearance of future regular cast member Mayim Bialik as Amy Farrah Fowler.