Roommates Dr. Leonard Hofstadter (Johnny Galecki) and Dr. Sheldon Cooper (Jim Parsons) have settled into their lives as Caltech University researchers. Timid Leonard explores experimental physics while the obsessive-compulsive Sheldon pursues theoretical physics. Their friendship is cemented on Sheldon’s nitpicky “Roommate Agreement” and a mutual adoration of nerdy things.
The series opens as Penny (Kaley Cuoco), an aspiring actress from Nebraska, moves in across the hall. Leonard’s instant infatuation leads him on a disastrous attempt to woo her. Sheldon sneaks into Penny’s apartment in the middle of the night to organize her clutter. Despite their awkward encounters, Penny agrees to go out with Leonard at the end of the season.
Always by their sides are friends Howard Wolowitz (Simon Helberg), a sex-obsessed engineer that lives with his mother, and Dr. Rajesh “Raj” Koothrappali (Kunal Nayyar), an astrophysicist that can’t speak in front of women unless he’s drunk. The four friends’ ongoing relationships and personal struggles are the glue that holds The Big Bang Theory together.
Leonard and Penny’s first date crumbles. Devastated, he first turns to his co-worker Leslie (Sara Gilbert) and then a doctor for comfort. Neither attempt goes well, though Howard has a short “friends with benefits” stint with Leslie himself.
Meanwhile, Raj fails his own quest to woo Penny, and Sheldon goes through an equally disastrous attempt to learn to drive. Events come to a head when Penny agrees to a date with the owner of a comic-book store, Stuart Bloom (Kevin Sussman). She realizes her deep feelings for Leonard just in time for the boys to be shipped off to the Arctic to explore a scientific theory of Sheldon’s.
Leonard returns from the Arctic and walks right into Penny’s desperate arms. Sheldon is not so pleased. His latest theory was a failure, and the boys have to travel to Texas to bring him back home after he tries to run from his troubles. Leonard and Penny’s relationship falls apart after Leonard says, “I love you,” before Penny is ready.
A double date set up by Penny leads to Howard meeting Bernadette Rostenkowski (Melissa Rauch), a sassy microbiologist. Sheldon develops an ongoing rivalry with actor Will Wheaton. Wheaton makes numerous guest appearances in the series that you can catch when you watch The Big Bang Theory online. An online dating profile Raj and Howard make for Sheldon brings up Amy Farrah Fowler (Mayim Bialik), a peculiar neuroscientist. They hit it off, and Raj and Howard are left to think about what they’ve done.
Sheldon and Amy’s first date, while strange, proves their compatibility is undeniable. Howard rekindles his relationship with Bernadette, and a one-night stand with Raj’s sister Priya (Aarti Mann) develops into a relationship for Leonard. Penny tries to act casual, but Bernadette and Amy see right through it. The girls bond over helping Penny through her feelings.
Howard decides to retire his womanizing ways and propose to Bernadette. Bernadette accepts and even forms a bond with Howard’s domineering mother. While Howard basks in success, Leonard is distraught when he learns Priya is moving back to India. Penny confesses to Raj that breaking up with Leonard was the biggest mistake she ever made.
Things aren’t going so well for Penny. Leonard finds out that she, when drunk, tried to have sex with Raj, and her acting career is going nowhere. Between that revelation and Howard learning about Raj’s ongoing crush on Bernadette, Raj finds his ways under the spotlight. Sheldon’s commitment to Amy is tested when Amy goes on a date with Stuart, leading Sheldon to propose a “Relationship Agreement.”
Leonard tries a long-distance relationship with Priya that falls apart when he learns Priya cheated on him. He impulsively asks Penny out again, and Penny accepts. Howard redefines the meaning of “long-distance relationship” when he learns he’s being sent to the International Space Station. The friends come together to throw Howard and Bernadette an impromptu wedding.
Howard’s experience in space is less than glorious. He’s bullied, homesick and stir-crazy, and his return home is equally depressing. Raj had introduced Stuart to the group as a temporary replacement Howard, but Sheldon eventually boots out Stuart.
Penny and Leonard’s relationship goes through its usual fluctuations, but in a stint of jealously, Penny blurts out, “I love you.” With their relationship feeling concrete for the first time, Leonard tells Penny that, because she has the most trouble with commitment, it falls on her to propose marriage.
Raj finally gets lucky in love, meeting a socially anxious girl named Lucy. It moves along beautifully until Raj tries to introduce Lucy to his friends. Overwhelmed, Lucy breaks up with him. Penny consoles Raj as he breaks down, and Raj discovers that he can finally talk to women without being drunk. The season ends with Leonard taking a temporary assignment on a ship in the middle of the North Sea.
Sheldon and Penny bond over missing Leonard, who surprises Penny when he comes home early. The joy doesn’t last long. They learn that a drunken Penny had married one of her ex-boyfriends in Las Vegas. Penny annuls the marriage, but she later gets drunk once again and proposes to Leonard. She takes his discomfort at agreeing while she’s intoxicated as rejection. Leonard offers to support Penny so that she can pursue her acting career full time. Eventually, the engagement is made official.
Raj scores another steady thing with a girl named Emily. Sheldon is the only one left feeling empty-handed. Despite enjoying his first kiss with Amy, he doesn’t know what will happen to him. After Leonard and Penny’s engagement, the comic book shop burns down and the university refuses to let him change fields. He leaves town to figure out what he’s going to do.