How Would You Describe the Plot of TBBT?

I watched the entire show, and I'm trying to sum up what it's about for a friend, but I have no clue how to summarise the plot whatsoever.

Edit: thank you for the answers, some have been very funny and also helpful. FYI to anyone else, I know sitcoms don’t have an overarching plot, but if you had to put the show into one sentence, how would you do so?