
Mensa-fied best friends and roommates Leonard and Sheldon, physicists who work at the California Institute of Technology, may be able to tell everybody more than they want to know about quantum physics, but getting through most basic social situations, especially ones involving women, totally baffles them. How lucky, then, that babe-alicious waitress/aspiring actress Penny moves in next door. Frequently seen hanging out with Leonard and Sheldon are friends and fellow Caltech scientists Wolowitz and Koothrappali. Will worlds collide? Does Einstein theorize in the woods?

Bernadette and Wolowitz leave their kids for the first time; Penny and Leonard try to keep a secret; Sheldon and Amy stick together; Koothrappali makes a new friend; the gang travels together into an uncharted future.

Sheldon tries to help Penny understand Leonard's work; Wolowitz gets jealous when Leonard starts hanging out with his new girlfriend.

Amy and Sheldon host a Valentine's Day edition of Fun With Flags; Penny and Leonard wrestle with getting older; Howard and Bernadette make a discovery in their hot tub; Raj is torn between Emily and another woman.

The gang celebrates news of Bernadette's pregnancy by singing karaoke; Wolowitz's reaction to the pregnancy announcement worries Bernadette; Wolowitz believes he needs to make more money to support his growing family.