When geeky Leonard and Sheldon meet their new neighbor Penny, Leonard is excited about a possible romance, but Sheldon is less enthusiastic feeling his friend is setting himself up for disappointment.
Leonard volunteers to sign for a delivery, hoping to make a good impression on Penny. But when he enlists the help of obsessive-compulsive Sheldon, his attempt at chivalry goes horribly awry as Sheldon takes it upon himself to clean her apartment while she sleeps. With Penny infuriated at the intrusion, Leonard tries to make amends.
Leonard is bummed to learn that his crush, Penny, is seeing someone. Encouraged by his friends and fellow scientists, Leonard actually summons the nerve to ask a nerdy female co-worker out on a date. With that under his belt, he musters the courage to ask Penny out too, but she doesn't realize it is a date.
Sheldon gets fired and decides to explore life beyond physics. His initial thrill of shopping with Penny devolves over time to him weaving ponchos as a shut-in, forcing Leonard to call on Sheldon's mother.
After sensing his pursuit of Penny isn't going anywhere, Leonard agrees to an unexpected sexual encounter with fellow scientist Leslie Winkle.
Leonard, Sheldon, Wolowitz and Koothrappali dress up for the Halloween costume party being thrown by their hot neighbor, Penny.
When Wolowitz seduces Penny's slutty friend who then takes over her apartment, Penny sleeps on Leonard and Sheldon's couch. With Wolowitz preoccupied, the guys discover they are without a fourth member of their group for Halo and in desperation, they ask Penny to join the game.
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.
When Sheldon and Leonard are invited to present their joint findings at a physics conference, Sheldon refuses so Leonard decides to go by himself. Penny steps in to mediate, but only causes a greater rift between the two best friends.
After hearing Penny sing, Leonard lies to avoid seeing her perform, but Sheldon takes it one step further with a more elaborate lie that leads to the appearance of his non-existent cousin.
When Sheldon gets sick, Leonard and the boys know what's in store and avoid him like the plague, leaving an unsuspecting Penny to nurse the world's worst patient back to health.
When Leonard and Sheldon meet a 15-year-old physics prodigy, Sheldon is distraught that this boy genius is younger and smarter. In an attempt to restore his sense of purpose, the gang comes up with a solution to derail Dennis' research. During his welcoming ceremony they bring in a slew of pretty young girls hoping he will be distracted from his purpose and they succeed beyond their expectations.
Since Sheldon's only focus is to prove his mental superiority while preparing for the Physics Bowl, the guys kick him off the team and enlist his nemesis Leslie Winkle.
When Leonard and the guys buy a time machine prop from a classic 1960 movie, its arrival creates problems for Penny and she tells the boys that their obsession with comics and toys is "pathetic." Stung by Penny's outburst, Leonard decides to give up his time machine and other geeky pleasures for Penny, only to learn that she has a date.
When Leonard, Wolowitz, and Koothrappali all become smitten with Sheldon's beautiful twin sister, Leonard devises a plan to eliminate his rivals. However, it backfires when Sheldon declares he will decide whom his sister dates.
Penny learns that it's Leonard's birthday and since he's never had a party, she decides to throw him a surprise party. Howard fakes a reaction to peanuts to get Leonard out of the apartment while the others set up, but when he can't get Leonard to stay he actually eats some peanuts and needs the medical attention. By the time they get back to the apartment, the party is over.
When Leonard finally asks Penny out on a date and she says yes, they each seek the "wisdom" of reluctant advisor, Sheldon. Despite reluctance on both sides, they end up going out on their date after a preemptive kiss.