What happens when hyperintelligent roommates Sheldon and Leonard meet Penny, a free-spirited beauty moving in next door, and realize they know next to nothing about life outside of the lab. Rounding out the crew are the smarmy Wolowitz, who thinks he's as sexy as he is brainy, and Koothrappali, who suffers from an inability to speak in the presence of a woman.
Amy gives Sheldon an answer to his proposal. Howard and Bernadette struggle with some unexpected news.
Leonard angers the university – and the entire physics community – after he gives an embarrassing interview. Amy and Bernadette bond over having to hide their success from Sheldon and Howard.
As Sheldon stresses about picking a wedding date, Amy tries to convince him he has a more laid-back side. Raj and Stuart compete to win the heart of Bernadette's new co-worker, Ruchi.
Howard and Sheldon bond when they drive to the desert to shoot off a model rocket. When Leonard's mom finds a new best friend in Penny, it rubs Leonard the wrong way.
Sheldon and Raj confide in Bernadette when they can't handle Amy and Howard working together. Penny and Leonard learn new tactics for how to deal with Sheldon from an unlikely source.
Sheldon goes head-to-head with Wil Wheaton for the role of the new Professor Proton. Penny steps in to take care of Halley when Bernadette and Howard both wind up on bed rest.
Sheldon and Bert collaborate on a research project, despite Sheldon’s embarrassment. Penny and Bernadette coach Raj on how to navigate his relationship with Ruchi.
Leonard and Howard are furious after they learn Sheldon went to work with the military behind their backs. While Bernadette is on bed rest, she asks Raj to do some digging when she suspects Ruchi is trying to steal her job.
Sheldon tries to teach the guys a lesson after they cut him out of a potentially valuable Bitcoin investment. A seven-year-old video reveals a secret about Leonard and Penny’s relationship.
Sheldon and Amy try to eliminate stress from wedding planning by applying math to the process. Raji 'breaks up' with Howard after realizing his best friend is actually hurting his confidence.
Sheldon and Howard plan birthday celebrations for Amy and Halley, respectively. Leonard receives a Christmas letter from his brother that causes him to spiral out about his own lack of accomplishments.
To discover who would be most qualified to be best man and maid of honor at their wedding, Sheldon and Amy subject their friends to a series of secret experiments. Penny reveals her true feelings about Amy.
When Sheldon kicks Amy out to work solo, she and Leonard bond during a series of science experiments. Bert the geologist replaces Howard in the band Footprints on the Moon. Sheldon finds Penny a surprising source of scientific inspiration.
Raj finds himself in the middle of domestic drama when he learns the woman he's dating, has a very upset husband. When Sheldon rents his old room back for a quiet place to work, he drives Leonard crazy by being a model tenant.
Leonard's book raises questions. Amy guest stars on the new Professor Proton show.
When Bernadette won't go into labor, the gang tries different tactics to get things started. Howard confronts Bernadette after Amy accidentally reveals she's already chosen their son's name.
Leonard jumps through hoops to help secure the perfect wedding venue for Sheldon and Amy. Also, Bernadette and Wolowitz have a hard time deciding who should stay at home with the kids and who should go back to work.