Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.
The Big Bang Theory is a comedy series about four young scientists who know all about the world of physics, and one girl, who gives the physics world a real spin.
Leonard Hofstadter is a smart guy who tries his best to complete his research and become famous for his work while also attempting to be the most socially-interacting guy in their friend group. Sheldon Cooper is one of the world's smartest men with an intellectual capacity through the roof and a language with scientific words normal people only have one comment to ('What-). Howard Wolowitz is an engineer with his mind partially on science, but mostly on women. He always tries his best to impress a girl and get his share of sexual intercourses, but he always manages to screw up somehow, probably with his bad knowledge of treating women in a bad matter. Rajesh Koothrappali is an Indian scientist performing experiments on black holes, outer space, life on other planets and not to mention string theory. Raj is mostly known for his Selective Mutism, in other words, his fear of speaking to women, making social interaction difficult for him. Finally, there's Penny.
Penny is the gorgeous girl next-door to Leonard and Sheldon's apartment, and though she does not have any knowledge in physics or science, she makes success by being a funny character frequently having hilarious comments and on- and off-going relationships.