I only just started watching this over Christmas and I devoured the first three seasons in record time. It is such fun---populated with highly quirky characters and sophisticated writing, and enough Star Trek, Star Wars and Marvel comic book references to satisfy everyone. We have all three (123 ) of the seasons.
Leonard and Sheldon are both brilliant physicists---colleagues, best friends and roommates. They are also friends with their Cal Tech colleagues, mechanical engineer Howard and astrophysicist Raj. They can explain the workings of the universe, but these 4 self-professed nerds know little about the real world. This is not a typical 'let's make fun of the nerd' show---the characters and the audience are treated with a great deal of respect.
While you're at it check out the non-fiction series 'Through the Wormhole'. Hosted by Morgan Freeman, this excellent 8 part series explores the deepest mysteries of existence and the questions that have puzzled mankind for eternity. Who or what are we? Are we alone? How did life begin? What happened before? Are we real or a computer simulation and how would we know? The series brings together the brightest minds and best ideas from the very edges of science to reveal the extraordinary truth of our universe. I am not through it yet- I find I have to stop and digest each episode for a few days after I watch it, but I have sure enjoyed the five I have seen so far.