Leonard and Penny's relationship resets to friendship after their date doesn't click. Sheldon is relieved by the return to normalcy.
Penny dates a new guy and Leonard rebounds with a woman from the Renaissance Fair. Both attempts at moving on are equally awkward.
Penny gets addicted to an online game and the guys are thrilled to have her in their world — until her gaming habit takes over.
Raj gets published in a magazine and fame goes straight to his head. The guys try to bring him back down to earth.
Sheldon has to learn to drive because his carpool situation falls apart. The driving lessons are a hazard to all of Pasadena.
A grad student develops a crush on Sheldon and starts following him everywhere. Sheldon is the last to realize what's happening.
Penny bans Sheldon from her apartment and he escalates the conflict to legendary levels. The roommate agreement gets creative.
Leonard dates Penny's friend and a Star Trek prop causes relationship tension. The game Rock-Paper-Scissors-Lizard-Spock makes its debut.
Sheldon inserts himself into Leonard's relationship with unsolicited advice. The apartment tension between logic and emotion heats up.
Leonard's girlfriend starts making changes to his life and apartment. Sheldon notices the warning signs before Leonard does.
Penny gives Sheldon a napkin signed by Leonard Nimoy for Christmas, and his emotional reaction is one of the show's most beloved moments.
The guys build fighting robots and Kripke challenges them. The robot battle reveals more about their egos than their engineering skills.
Sheldon tries to make a new friend using a scientific algorithm. The friendship flowchart is peak Sheldon Cooper.
Penny borrows money from Sheldon when she's short on rent. Leonard's feelings about the arrangement reveal deeper dynamics.
Leonard's mother Beverly visits and her cold analytical style makes everyone uncomfortable — except Sheldon, who finds a kindred spirit.
Penny accidentally ruins one of Sheldon's couch cushions and pays a steep price. His attachment to his spot is the stuff of legend.
The guys take a train trip hoping to meet a celebrity. Howard's bold approach to flirting is as confident as it is unsuccessful.
Penny starts a business making hair accessories and Sheldon helps optimize her production. His efficiency obsession meets her creative energy.
An attractive new neighbor moves in upstairs and the guys compete for her attention while Penny watches with amusement.
Leonard goes to the comic book store and ends up on an unexpected date. Penny reacts to his new romantic interest.
The guys hit Las Vegas and Howard's overconfidence leads them into some interesting territory. What happens in Vegas barely stays there.
Howard's classified government project hits a snag and the guys try to help fix it despite lacking security clearance.
The guys are offered a summer expedition to the North Pole. The decision to go or stay splits the group, and Penny and Leonard share a meaningful goodbye.